No, you can't keep a "backup" of the expansion data in internal memory.
All you can do is keep a copy of the installation file that you generated from YEM on either your computer's drive, or a USB. If you want to go back to a previously loaded set of expansion data, you need to re-install the previously saved file. This will replace everything.
However think you're asking about being able to keep what was previously there and add to it. There are 2 options here.
+ Create a new pack in YEM that has everything you want in it by ticking the boxes on multiple packs
+ As above but use 'Quick install' to create a kind of 'topup' file with just the new data. (Personally this has never worked properly for me, though it has for others)
In either case remember the max total size is 3GB.