I'm sorry I'm not exactly sure what you are asking.
I think you are asking if you can add 2 more batteries to your service/ house bank. Which of course you can. But if you current batteries are older you might want to change them all.
Then I think you are asking if you can add a lower amperage battery to your current Genset battery to increase your capacity. Which you can. But it might be better in the long run if both batteries were of the same capacity.
Hope I got some of that right.
If not don't hesitate to let me know.