I use to have a pair of Hella Black Magic 500s (55w) wired up to my factory fog circuit. I simply cut the male adapter off the end and spliced the wires together. Worked like a charm for a couple years up until I got rid of them. Now I have a 30" led bar spliced to one side of the fog circuit. Still no problems. If you go with halogens and they are more than 55w I would suggest wiring them up separate. You risk burning up wire (fire) and/or blowing the fuse. In which case, you'll be doing double the work from fixing your fog circuit AND running new wire.
Same concept with higher wattage halogen bulbs in headlights. I can't tell you how many times I've seen people melt their headlight harnesses cus they didn't beef up their wiring after putting in higher wattage bulbs.