You could try to reuse the holes but:
A) They're a bit naff.
B) Often when the original spoiler is ripped off (speedbump/carpark/kerb I always assume) the holes are ripped/destroyed too.
C) Good luck working out what was a splitter hole, what is a random stone-chip hole, what is an injection-moulding-release hole & what is just dirt lol.
A much better idea is to get some short "self tapping" screws from B&Q (failing that short wood screws seemed to work fine) and just screw those straight through the splitter and straight through the bumper. The splitter is made from cheese and the underside of the bumper is hardly difficult to screw a thread into.