I fear the short answer is no.
Despite both series use a shared backplane stacking approach (dedicated backplane stacking modules and dedicated stacking cables), I think they have the implicit requirement - for successfully building the fabric stack - to use models that all share the same software line (so, considering Aruba 2920, WB with WB or, considering Aruba 2930M, WC with WC, not mixed WB/WC should be permitted)...the requirement can be translated into "models that belong to the specific same series".
Edit: for ones that will ask...but...yes, both Aruba 2930F Switch Series and Aruba 2930M Switch Series share the same WC code software line...yeah...BUT they don't share the same stacking approach (VSF Virtual Switching Framework versus Hardware based Backplane Stacking)...so at the end...no...neither Aruba 2920 with Aruba 2930M nor Aruba 2930F and 2930M can't be mixed, as members, in a single fabric stack.
