If I have a NinjaTrader key already. Can I use it or must I use yours?

Yes, you can use your NinjaTrader key. The benefit of this is you can trade your own funded account with NinjaTrader and your evaluation and paid/funded accounts with Apex.  You do not need a multi-broker for this.

With our NinjaTrader key, you can only trader the Apex Trader Funding evaluation and paid/funded accounts.