Buyers closing costs are exactly that: costs that the buyer is responsible for. As a generous incentive some sellers are offering to pay the closing costs on behalf of the buyer. In the builder's case, often his company has a relationship with a lending institution. This relationship allows the builder to recoup some of the loss he is suffering through paying for the buyers closing costs.
In any case, the buyer is free to choose any lender. If you want to participate in the savings afforded to you by using the "builder's lender" you should choose the builder's lender. No one is forcing you to do anything in this transaction.
Possibly, a builder would extend his offer to include another lender if he was so inclined. But, you can not "force" him to pay your closing costs regardless. Successful negotiations are characterized by a win win situation. No one truly wins under any other scenario.
Happy house hunting. - Fri Apr 13, 2012