I would recommend replacing it with something like SAP_DUMMY which can at least read and match code on 1001 and 0105... (the PLOG sort of things).
As Ronen mentioned, you should change this BEFORE you restrict some system type user's access as they don't have personnel numbers.
Which BADI are you referring to? If it is in a central API then you might be OK. If not or there is hardcoding outside of the reserved API then you could be in for surprises.
Cheers,
Julius