Well, if you absolutely cannot return it and it is absolutely too soft, your only real option is mattress surgery. Take out the cheap foam on top, add some latex in it's place and voila!
However, I don't think anyone here has yet done mattress surgery on an individual-coils type mattress (I assume that's what it is?), so you will be blazing new territory. Budgy might be able to tell us something about those springs that you have and whether or not it is "foam encased" around the edges. If it is, that's yet another problem. But honestly I think you have nothing to lose at this point by opening 'er up and seeing if you can repair it by replacing the foams.
You can't make a too-soft mattress firmer by adding a firm topper. Putting something underneath it won't help unless your box springs is sagging.