Just finishing my Prusa i3 build, and realized that my Ramps board w/lcd control panel doesn't have usb adapter. Turns out I do want to be able to connect to my pc via usb. Ok to solder an adapter in now, and how exactly do I go about it?

The USB interface is not on the RAMPS, but on the Arduino Mega 2560 which you need to drive the RAMPS (and also your LCD). So if you don't yet have the Mega board, you'll have to get one...

Aha, got it. Thanks