Would a whore let you phuck her if you dressed like a ninja? If you had the cash, she sure would. Prop firms operate the same way. If you've got a checking account with the funds to pay the firm's deposit and training fees, they'll let you use Ninja trader.
Only if Ninjatrader can access your prop's API can you use them. Do you see any props on the list: http://www.ninjatrader.com/partners