good question. I will like to know it for future preference as well.
But as what I understand, it is not a good idea to cut a key from third party. You suppose to cut the key from dealer, b/c all the key are encrypt with security chip. With third party key, you could only start up the engine once.
hmm.. but cut key from dealer is also more expensive.................
... but reconsidered when I thought about how a third key fob would work.
Yes a third key and fob can be obtained from the dealer. However only two fobs can be set to the seat memory positions. After second thoughts we decided it was more of a hassle and cost than benefits. But it can be done.
You can program the remote yourself (do a search for threads on howto), but not the key itself. I know because I lost a key and a remote and had gone through the process of replacing them. My local dealer wanted an arm and a leg for them, but Tim will replace and program the key for $25, and the remote for $85. You'll need to send in a key for the reprogramming; I sent my valet key.
Someone mentioned that any Acura or Honda remote can be programmed to work on MDX, though I haven't try that myself. If so, you can probably get one from EBay real cheap.