you would have to match the voltage and amperage,millivolts output i think,and youd have to know whether to wire it in series or parallel,ive often wondered the same thing,and its entirely possible to do,maybe another macbook battery might fit,out of a plastic macbook or ibook -mba or something,now how it charges might be tricky,as it might overdraw the watts/external transformer,overheat it,but,if its in series,it may just take a lot longer to charge,it could be done im sure,the mba has 3-4 seperate batteries,does it not?
youd need to work out which wires are power in[charging]and power out too,and do some rewiring offa the plug to battery.