how to change the skill "possess"

The fight command eventually reaches C2/1610 as well and as it is now you're jumping over the "Character executing one hit" routine at C2/317B as well as at least one other routine. You also forcing magic as the command to execute for every character.

I suggest you put the following at C2/1635:

Code:

20 69 64       JSR $6469

Then at C2/6469:

Code:

C2/6469: C9 06    CMP #$06
C2/646B: D0 08    BNE $6475 (branch if not capture)
C2/646D: A9 C6    LDA #$C6
C2/646F: 85 B6    STA $B6
C2/6471: A9 02    LDA #$02
C2/6473: 85 B5    STA $B5   (in case $B5 is used elsewhere...)
C2/6475: 8D 13 34 STA $3413
C2/6478: 60       RTS

I haven't tested this though, there might be issues I haven't foreseen.