Regarding the use of a mattress topper with a pillow top mattress

Hi,
I wrote this question to you the other day, "My pillow top mattress is sagging in the middle. Can I put a mattress topper on top of the pillow top mattress or will that be too much?" The mattress in question is not my own. I misunderstood the problem. The pillow top mattress is not sagging, but it is lumpy. It was described to me as feeling like there is a steel bar in the middle of the mattress. (There is not an actual bar.)