No, BitcoinBuilder is a third-party site where people can exchange GoxBTC for RealBTC. It works on a standard exchange-basis, with bids and asks, so the exchange-rate is set by the market. Since internal transfers between Gox-addresses are still possible, users can deposit their GoxBTC on BitcoinBuilders MtGox account and vice versa, enabling this type of exchange.

mcxNow is completely unrelated. It's an altcoin exchange. You get some interest on your deposit, which is paid for by part of the trade fees collected, but this amount is incredibly low. It's not worth the risk of leaving your coins in an exchange. In addition, the operator of mcxNow has an incredibly dubious track record. While the site functions reasonably well, I wouldn't trust it with my coins any longer than the time needed to make a trade.