This tutorial can be used for ANY acoustic- to- electronic drum conversion with minimum alteration needed.
It contains detailed images and instructions that should explain the process in a way that many words may not.
I am learning how to do this as I go, so if you have better ideas please share them!.
I have been working on building electronic sets for only a few months and sometimes I figure out better designs after the prototype that I've built is finished. That said, if I find a better way to tackle a project, I will post an update to save everybody else the part of having to find out the hard way like me. As any other Diy'er out there knows, the internet and sites like this are what we use to learn and sometimes teach each other, I give credit to the authors of sites that I used to learn and design this and other tutorials posted in YouTube under the user name Lucyferina.
Feel free to ask any questions about this mode or any ideas you might have, if I know the answer I will gladly answer and if not, at least will try to point you in the right direction.
Happy drumming!!

To see a video explaining this concept:
copy and paste the link below in your browser's address bar and press enter.

http://www.youtube.com/watch?v=YAbrT8bZP7g

One last thing..... If you need to see a bigger version of the pictures click on the "i" icon on the left top corner of the picture, this allows you to select a bigger version and read the text that I added to them.

Step 1: Create a Bridge

This First step is to Create a bridge for the head piezo that should span from one side of the drum to the other. in order to maintain the drum integrity, the bridge is mounted to the drum utilizing the screws that hold the drum lugs and "L" brackets as shown in the accompanying image. In order to prevent double triggering from the rim piezo, rubber washers can be utilized between the "L" bracket and the lugs as well as the bridge itself.

Step 2: Mount the Head Piezo to the Bridge

Once the bridge is installed, the next part is to mount the piezo sensor on top of the bridge, in the case of Roland modules, the piezo should be sandwiched between the bridge and the foam cone by double sided tape,( For proper sensitivity, the piezo sensor gets positioned wire side down facing the double sided tape and the copper only side facing the foam) I don't have a Roland module to confirm this but in the Roland forums people agree that the head doesn't have a lot of contact with the foam to get a better response.
In the case of Alesis modules a foam block is used instead. Roughly a 2 inch by one inch rectangle, this should be big enough to cover the piezo which gets positioned below the double sided tape between the head and the foam block wire side facing the foam block. in my case, I put a piece of double sided tape on top of the foam, then the sensor cable side facing the sponge block, then another piece of double sided tape on top of the piezo facing the drum head, also in my case the piezo turned out to be too sensitive when placed in the center so I ended up placing it closer to the rim (Roughly 2 inches away from the drum wall) This allowed me to create a mini bridge which only uses a wood square of roughly 4 inches by 4 inches and a single "L" bracket to hold the head piezo. This also help utilize less cable to wire the sensors when creating a double zone drum.

Step 3: Add Rim Piezo Sensor

The next step is to add a piezo sensor for the rim of the drum which will allow us to play a snare rim if using a snare or cymbals, cowbells, or other percussion sounds, or any other samples that your module or software will allow.
A breakdown on modules that I know:

Alesis DM5 does not allow double zone but does have enough trigger inputs (12) that you could wire a drum a little differently and actually have a double zone drum (Or cymbal).
the DM5 does have MIDI out which allows it to trigger software. a MIDI to USB adapter is needed to connect it to a computer.

Alesis i/O it has 10 inputs all of them double zone (When using a TRS cable) TRS picture on the following steps. This is not a drum module but instead a Trigger to USB interface, it does NOT produce any sounds but instead allows you to play software like BFD2 , Superior Drummer, EZ Drummer, Steven Slate Drums (which is the one I have) and more similar programs, The programs just mentioned are far better than any stand alone module due to the variety of sounds that you can get from any drum (some have over 90 velocity layers) making for a very realistic drum experience which mimics the real thing so closely you'll wonder how you played anything else before.

Roland Almost all Roland modules that you can buy today have MIDI out capability and at least some double zone inputs, more modern modules even have triple zone capabilities and even MP3 inputs to play along with your favorite tracks and some other fancy features that make them more than capable to support any electronic drum kit.

My personal preference is the Alesis I/O because of it's price and capabilities, it doesn't support triple zone but the only part where I would need it is for the ride cymbal. I am working on a way to do a triple zone cymbal on a double zone input, I think it has been done by racer52online in YouTube under "DIY Dual Zone Cymbal with Choke.MPG"

Step 4: Wiring the Piezos for Head and Rim Triggering

The picture included shows the proper wiring for the Alesis modules, this should work for the Roland and others but if in doubt you can look up on your module manufacture's forum for the particular wiring of the piezos. Again in the picture you see the recommendations for the different foam (cone or block) and also how far both the cone and the block need to be above the head level in order to offer resistance and provide good sensitivity to the piezo.

Step 5: Wiring Explained

Simple image that shows how to properly wire a double zone snare, or tom, this can also be used to wire a cymbal for bell and bow.
That also shows what a TRS connector looks like.