Plenty of sites take USD (I assume you mean US Dollars?). Check out this link for a list of sites accepting US real-money players: http://www.cardschat.com/us-poker-sites.php
Granted, you probably won't be able to deposit using a credit card, but most sites will accept the All Acces Visa
Giftcard (the non-reloadable type), that you can find at any Wal-Greens, and some grocery store chains.
As far as your question goes, you need to read each individual site's Terms and Conditions. While some sites allow you to withdraw your freeroll winnings carte blanche, many (ok, most) sites have a raked hand requirement, or the condition that you make a deposit before withdrawing freeroll winnings.