As this is qhimm, I guess you are talking ff7?
First of all, the displayed damage is 16 bit but holding negatives so we can go up to 2^15 -1 which is 32767.
Now extending this is technically possible, there are multiple variables of different size holding the damage during the calculation. With a strong missing score shot the game must be able to handle several hundred thousands of damage which it is, on psx the threshold for base damage goes as far as 2^18, on PC, though I'm no expert for the port, should be even more.
Now the tough one is altering the code. One has to change the memory assignment for the specific battle routines to make it use larger variables for the whole calculation. That should be damn. hard and afaik require a pretty complete rewrite of the battle system.
For starters take memory dumps to get to the files needed to execute the damage calculation. If you rock you will find the correct code, disassemble it, rewrite it, make the rest work with it, repack and release.
I don't see that happening though