how to add a couple force to i,j particles?

it seems that the command can not add a force on j atom, although I indeed write a code such as f[i][0] += -addforce.

On that note where the atom j in the neighborlist is a ghost for another processor, you must define a packing routine for the reverse communication of the force you perturb; see the virtual function pack_reverse_comm in fix.h for the declaration. It’s possible you can find an example of this in another fix being implemented.