1) Is it possible to plug a microphone into a bass amp? - Yes, you can plug a dynamic microphone into an amplifier, but there will be a signal/impedance mismatch, as there is a difference between the output level of a microphone compared to an electric instrument pickup. However, I don't know why you would do this with a bass drum. It would be better to run the mic on the kick drum into a microphone preamp on your PA if it is not loud enough.
2) Your friend might be talking about using a speaker as a microphone, which can also be done, but it requires wiring the speaker in reverse. A speaker is very similar to a moving coil dynamic microphone, in that it has a diaphragm (cone) and a big magnet. If you put the speaker in front of your bass drum and wire it to a microphone preamp, you can record the drum. This is the principle behind the Yamaha Subkick. You can also do this with headphones - plug them into a microphone input and yell into the earpiece - it will transmit sound. Kinda cool and lo-fi.