Can I use MCP3002 for undersampling?
Look at the LTC2285 used for this sort of thing: -



It can run at 125MSps flat out but it has a 640MHz sample and hold - this tells you what the main problem is when undersampling - you need a really fast sample/hold circuit to ensure you "capture" the detail in the much-higher-frequency carrier wave.

Even the lowly LTC2245 with its 10MSps throughput has a sample and hold that is rated close the 2255: -