Yes the 958c DI Combo (or the new 958c HD DI Combo) units will both use the LakeMaster and Navionics Premium South mapping cards.

I do not know when LakeMaster will be adding Lake Martin to their covered lakes for Alabama. If you did not fill out a lake request, you should do so now (and get all your fishing buds to do so too!):
http://www.lakemap.com/Request-a-Lake/

I don’t know if LakeMaster will offer a mapping card upgrade or not. You would not have to buy the LakeMaster card today; you could wait until Lake Martin was added to buy it. Mapping cards can be added at swapped out at any time.