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If you think you'll end up upgrading other parts of the system later on, you might want to look into putting a new headunit in there, although the swap from the 460 system seems to be a bit of a chore from what I understand (I didn't have to deal with it in my car).

But yeah, you can just add the subs/amp to the existing system if you prefer, or if you're on a tight budget.

Good luck!
line out converters do the job, but the stock ford systems kinda suck for tapping into for subs. ford dials out a little bass when you turn the volume up high, so you lose some output from your subs. if you want to do more with the system at some point, try to do at least speakers and a deck at the same time, that way to can bypass all the the existing ford crap and not worry about interfacing. just run new speaker wire from the deck to each speaker location.
You need a level converter, and if you tag it right it will sound great, you need to tag it off the passenger side amp in the rear, there is a plug with 4 wires two are black with green and two areblack with white, these are the four you use, it will sound just fine if you don't want to get rid of the factory head unit, good luck let me know if you have questions Dan