Yes, this is possible. You will first want to turn off auto-sync in iPod Preferences in iTunes (with your iPod attached to your Mac, select it in the source list, and then click the iPod icon button on the lower right).
Then, in iTunes, manually copy over MP3s from your iTunes library to your iPod. Once the MP3s are copied over, delete them from your iTunes library & delete them off of your Mac. By turning auto-sync off, the next time you connect your iPod to your Mac, it will not try to make your iPod look exactly like your iTunes MP3 library. So, iTunes won’t delete the tracks off of your iPod that are no longer on your Mac & no longer in your iTunes library.
Then import (or rip and encode) the next batch of MP3s you want to put on your iPod, manually copy these MP3s to your iPod in iTunes, then delete them out of your library and off your Mac. Repeat until you have all the MP3s you want on your iPod (or until your iPod is full!).