Hi cokewithvanilla,
An airbed uses the same type of materials in the comfort layers as any other mattress.
In the case of the sleep Number i* … you can see the comfort layers listed on their website here.
the mattress cover is quilted with 2 layers of some kind of fiberfill and an inch of super soft polyfoam.
Under this there is 3" of some kind of memory foam. They don’t provide any specifics about the type or density/quality of the memory foam. It’s quite possible that the gel memory foam that was used to replace it (assuming here it’s a version of gel memory foam and not some other type of gel foam) is higher quality than the original memory foam. You could see if it was gel memory foam by pressing it and seeing if it is a slow recovery foam (it doesn’t come back instantly)
Most larger manufacturers don’t provide details of their foam layers but you could always call them to see if they will provide you with enough details of the type and density of the 3" memory foam layer that you could replace it with a memory foam layer that was similar or better quality. If they do (which is very unlikely) then there is a list of better online sources for any type of memory foam, polyfoam, or latex in post #4 here.
If you call them at the number listed here … they will also give you a quote for a replacement layer of the same kind although you still won’t know the quality of the memory foam they are using.
It’s probably not a good idea to add an extra 3" of foam inside a cover that isn’t designed for it … assuming it would even fit in the first place. Adding an extra 3" of gel memory foam to an existing 3" of memory foam in the mattress itself along with the inch of supersoft foam and the fiber layers would give you a very thick comfort layer (with 6" of memory foam) and the risk of alignment issues that can come from your heavier pelvis sinking in too far would be very high IMO. Of course it doesn’t do any harm to try it but it certainly seems to me to be a risky construction. If you needed some extra memory foam on top it would be 'safer" to use a thinner topper. The thinner the topper … the less the risk of support and alignment issues.
Phoenix