In short i have no idee how this polymorph works.
Welcome to the club. My advice is to ban it entirely.
In any event, the physical form of a Warforged Charger give you two slam attacks, each doing 1d8+5 damage. Your BAB does not change, and both slam attacks are considered primary attacks, so you get two slams at +11 (+6 BAB, +5 Str).
Natural weapons never gain iterative attacks (monks being an exception), so you can't attack twice with each slam due to your +6/+1 BAB.
Slams are natural weapons, but a slam is considered an armed attack, so you don't provoke AoOs. Transforming into a Warforged Charger gives you its natural weapons, so no feat needed.