You can't mix Aruba 3810M (Hardware) Stack's member into an Aruba 2930F (VSF based) Stack and viceversa.
Using other words each Stack admits joining of Switches of the same family of its members.
Stack's members should belong to the same Switch series (either Aruba 3810M or Aruba 2930F or Aruba 2930M or Aruba 5400R zl2) and each Switch series is able to form Stack with its very same members (and that is true no matter the stacking technique is the same, backplane versus frontplane...so an Aruba 5406R zl2 VSF can't stack with an Aruba 2930F VSF and viceversa...the same is true between Aruba 3810M and Aruba 2920 or Aruba 2930M).
At best, once Stacks are formed, you can interconnect them as they were two single switches (which is true because a Stack shows itself to its peering devices as a single logical entity).