how to change the skill "possess"

No, Possess is one of the skills that its effects cannot be changed easily. The only thing you can do with it in USME is change around the attributes in the Battle Editor, and of course assign it to other characters in the Actor Editor, but to edit the actual skill itself, you would need to do some ASM hacking.

This is from the Co-opted Bank C2 Disassembly on assassin's page: http://www14.brinkster.com/assassin17/guides.htm

Possess

C2/17E5: BB TYX
C2/17E6: 20 C1 19 JSR $19C1
C2/17E9: A9 20 LDA #$20
C2/17EB: 0C A4 11 TSB $11A4 (Set can't be dodged)
C2/17EE: A9 A0 LDA #$A0
C2/17F0: 8D A9 11 STA $11A9 (Stores Possess in special effect)
C2/17F3: 4C 7B 31 JMP $317B

That is how it is currently, I myself cannot edit and/or create new skills from others like this yet, so I'm afraid I cannot help you any further. ;( All I know, is that this skill is very basic, so a lot of advanced hacking would be needed to make it into something else all together.