Word for "stepping stone" or "lemma"

Preparatory step seems to fit your meaning.

Also intermediary step.

Also vehicle: you can call this intermediary step a vehicle toward further advancement.

But, why would you not use stepping stone? It's a perfectly acceptable phrase for that sort of thing.

Update

In response to the context given in comments:

Preliminary excerise is another good fit.

Also, Formative.

Not quite a fit, but a really cool word. Progymnasmata, Greek meaning fore-exercises refers to a series of rhetorical exercises to prepare students for future writing tasks.

Prerequisite: something required beforehand.

Precondition: something that must come before or is necessary to a subsequent result.

If a step is required before another step can be taken or prepares for that next step, then the required or preparatory step is a precondition or prerequisite.

A prelude perhaps. I think it's well understood outside its musical context.

It seems to me there's two slightly different things here. One is a necessary thing that you need before going on to the next stage, as in stepping stone - some good sugestions in the other answers.

The other is more about familiarising yourself with the situation before trying it for real:

dress rehearsal / trial run / dry run