Can I substitute whole-wheat flower for spelt here?

Well, spelt IS in fact wheat so the quickest answer is yes, it can. But a better question is this: is the spelt flour made from whole grains or have the grains been broken down, as is usual with e.g. all-purpose flour, and the germ and bran discarded? If the spelt flour is whole-grain then whole-grain wheat flour is fine, practically the same thing.

Yes, in Good to the Grain, Kim Boyce says: "Baking with spelt is extremely easy and satisfying, as it can be substituted directly for whole-wheat or all-purpose flour in most recipes." -- so I think you could go the other way too.