Yes you can. Usually you have to disclose other funding sources (or Financial Aid will already know from the department). If you think you will need additional money to pay for fees not covered in tuition reimbursement, books, or living expenses, then call and talk to the Financial Aid office about what you should apply for.
You might want to try calling the Elena Hillenberg (her number is listed on the NSF EAPSI page) to see if she can give you an update on the status of your application.
I got my tentative acceptance email for Singapore just a few minutes ago with instructions to reply by 5pm on Thursday. Good luck to everyone else who applied!
@tropicaltrees - Thanks! That at least makes me feel a little less angsty. Keep me posted if you hear anything else from Singapore and good luck! Hopefully we'll all be seeing each other in DC at the orientation!
Not to sound like an idiot, but those who have gotten your tentative acceptances: were they email or snail mail? (I've applied to Singapore and haven't heard a peep yet).