Hi Jackstraw,
It’s certainly possible yes but it may be a somewhat risky option.
You can see my comments about choosing a firmer mattress first with the intention of adding a softer topper afterwards in post #2 here.
There is a list of many of what I call “simplified choice” mattresses in post #2 here and some of them include multiple firmness options (including softer versions). As you mentioned innerspring mattresses are less common in the online world than different types of foam mattresses (memory foam, polyfoam, latex foam) but there are a few options in post #6 here and the posts it links to that may be helpful.
Subject to confirming that any retailer or manufacturer on the list you wish to deal with is completely transparent about the materials and components in their mattresses (see this article) and to making sure that any mattress you are considering meets your criteria and the quality/value guidelines here … the better options or possibilities I’m aware of in and around the New York City area are listed in post #2 here.
Phoenix