Hi Diane37,
Anything is worth trying because your own personal experience is always more important than any theory but a layer of latex that soft underneath the mattress may affect alignment more than pressure relief or even “feel” so it would depend on the type of softness you were looking to change (see post #15 here).
If you are looking more for “a touch to a little” extra softness or to change just the surface feel then an inch or so of soft latex on top (or another material such as memory foam) would be in the range and a shredded latex or lanoodles topper may also do the trick with little risk for alignment (see post #4 here under shredded latex toppers). A featherbed or down alternative topper or mattress pad can also soften the “feel” of the mattress.
Phoenix