It is posssible, I will tell you that because I have many friends who do that (mix subwoofers). Each sub would need its own box and If you had just one box it would have to be custom and chambered to each individual woofers specs. I couldn't recommend it to you just because I don't know. That would be quite an interesting set-up though if you worked it out.
the most important part would be to make sure that the impedence you came up with aftre hooking them all up would not go lower than your amp can handle. I have seen people do it though every comany will tell you not to mix different Ohm value subs on one amp.