This is how it could be done.
But your Samsung SGH-P900 will not work mobile anymore ... be warned ! ;-)
Step 1: Disassemble the device
As soon as the screws are removed the device could be opened. There are some plastic noses that could be opened easiely.
I like to connect to the keyboard to be able to control the device remotely. But the connector or the keyboard has got very tiny connectors. No chance to solder here without destoying the device.
I thought it would be best to solder directly on the button pads. Doing that the keyboard will get unusable and the device could never be put into it's initial state.
This is the point of no return!