Depends on what you mean by an alternate IP address.
One possible interpretation is what's commonly known as multihoming: a single host has different IP addresses on different network interfaces. For example, one IP address on wired ethernet (en0) and a different address on wireless (en1).
http://en.wikipedia.org/wiki/Multihoming
If that's what you want, try googling:
multihome mac os leopard
That may seem like a brush-off, but frankly, there is a book at Amazon, posts on Apple discussion boards, and various articles that talk about it or describe products. Also, Leopard Server provides multihoming out of the box. I don't know which of those would serve you best, or even whether that's what you want.
If that's not what you want, then you'll have to explain what you mean by an alternate IP address, perhaps by explaining what you're trying to accomplish.