Can you use a serta boxspring with a simmons mattress?

It depends on your comfort needs. If the height provided by the boxspring plus the serta mattress is just right, it shouldn't be a problem. But make sure you are getting the right mattress size.

No, not if you want your warranty to remain valid. When I bought my Serta I thought of doing the same thing, but was told by the Serta sales person that doing so would void my warranty.