Hi TedLogan,
I’m glad you found us … and welcome 
A topper can be a very good option if your mattress is too firm and needs some extra softness and pressure relief as long as there are no soft spots or sagging in the mattress.
Post #2 here and the topper guidelines it links to will be useful in helping you make the best possible choice for a topper.
I would also be aware that if you add a topper over memory foam that it can affect the amount of heat that reaches the memory foam which can affect its feel and performance (it won’t soften as much) which can make it feel firmer. The most common topper for a memory foam mattress would be another layer of memory foam so you would still have the “feel” of memory foam that most people who choose a memory foam mattress would likely prefer.
Phoenix