i know it states: "both pistol and rifle customised to the specifications of the individual Assassin."

so does that meen i can get them to put a targeter into it ?

or is it only allowed what is on it already?

Only what's on it already. What's meant by the statement is that the weighting, grip shape, placement of reload controls, etc, have been customised (and for that reason it'd be really awkward for other people to use - albeit not as bad as the spy mask, which is hard-locked to a specific user).

It's already got a scope, so it can't accept a targeter as well.

As an observation; you're a vindicere firing exitus ammo. You really don't need a targeter.