If was able to subdue a magusus familliar could I use him as a penatration boost without haveing to penatrate its resistance.
In otherwords can I use it a a link to it's master by possession and add +4 to my pertration multi without haveing to go through its resistance..???
Thrumbar 
If you had possesion of a magus's familiar I would rule that you had a Fixed Arcance Connection, and so if you have the Penetration Ability you could take advantage of the benefits of an Arcane Connection. In this case that would mean you would get to mulitply your Penetration Ability by 4 and then add it to your penetration total.
Sweet. 
Familiars are actually listed along with talismans on the arcane connection table as examples of indefinite-duration arcane connections.