Without knowing what kind of hops they are, there is no way to tell how much bitterness they will add to the brew. Well, there is. But it would involve buying an acid titration kit and figuring out how to use it (a quick google came up with kits for testing the acidity of wine, but not much for hops), or sending some off to a lab (which costs money).
I would suggest using them for flavor or aroma hops, as in the ones you add towards the end of the boil, and continuing to use pellets for the first additions.
It might not be a bad idea to make a tea with them first, so you can see if you like the flavor or not.
As far as quantity goes, because pellet hops are pulverized, you get better utilization from them. A quick google suggested using 10 to 20 percent more whole hops than pellets.
