A trimmer mower also produces less dust helping people with bad allergies to grass (Like my wife).
Step 1: Buy an after market trimmer head and install it.
Look for one that DOESN'T spool out more line as needed (you may be able to get this to work, but I don't know.). And attaches to the trimmer with a bolt. It should also have a very round bottom, unlike a whacker this trimmer head needs to be able to support weight and glide across the ground.
I purchased and used this one.
Remove the blade from the mower, and see if the bolt head will fit through the trimmer head. If the head is too big (mine was) you need to grind off a little material from each edge of the bolt head so that it will fit.
With that done, simply screw the trimmer head on.
Balance is extremely important, so if you install longer trimmer lines, make sure they are the exact same length.
Step 2: Drain the Fuel Tank or Remove the engine from the Mower Deck
Step 3: Cut the front off the mower.
I used a cut off wheel for as much as I could but there where a few place I couldn't quite reach. I would recommend just removing the engine. It will make it MUCH easier.
For the places I couldn't reach with the cut off wheel I sharpened an old flat face screwdriver so it was like a small chisel and used it like a can opener to cut the deck. (The deck on my mower was very thin).
Step 4: What I would do differently next time.
The mower will be fairly flimsy when done. When I get time, I'm going to put some 1/4" steel rod around the outside edge where the deck was cut, and add a few stiffening supports.
I also want to put some longer string in it.
A real trimmer mower has larger wheels that are placed directly under the engine, this makes the weight balance nicer and keeps more weight off of the trimmer head.