What you would want to do is populate those empty spots next to the propeller chip with some headers so that you can make a snap-in "shield" oscilloscope board.
The "easy" way (well, easy as far as hardware goes) way to do A/D conversion with a propeller is to make a Sigma-Delta ADC
converter. Unfortunately, you won't be able to make one that works very well due to the distance from the pins. This means that you will need to add a discrete A/D converter IC—which will probably get you better resolution than a sigma-delta converter ever would anyway (depending of course on the quality of your ADC).
As for what would be a good ADC IC, I'm not sure off the top of my head. You want a sample-and-hold converter with something like an SPI interface. If you can find a dual-channel (or more...) version, then even better.
You'll want some sort of buffer in front of the A/D converter to protect it, at least a simple op-amp follower
. Be careful which component you choose for this because it will significantly affect the scope bandwidth.
There are LOTS of details that go into making a good
o-scope, but I'd aim first to make a snap-in board that works at low frequencies and then later look into making something with better performance.