George Fragos has made great tutorial for MaemoMapper. Details how to add new map repositories can be found:
http://fragostech.com/MaemoMapper/#repoman

Look down until you see the "Download" section.

EDIT: Once you find that magic "Download" button in MaemoMapper it will download you several map repositories, including Google maps. You can then later delete the ones you are not using. Also, if you plan on saving maps on the device you may want to change the default map database file location to memory card.