The Amiga generally has two different kinds of memory: Chipram and Fastram. Fastram is ONLY for the processor, and it's therefore faster, because it doesn't need to share timeslots.
Chipram is shared between the Amiga chips and the processor. This makes it slow if seen from the processor side.
Maprom has nothing to do with Chipram. It's a method to copy the ROM contents to Fastram, and then make it appear like it's ROM. This speeds up execution of many things, as Fastram is faster than ROM.
To expand chipram, the only chance is to exchange the Agnus chip with a board that has the 2Mbyte chipram Agnus and the required memory. The maximum that you can get is 2MBytes chipram.