Hi! if you mean that you need to ADD an Aruba 3810M (with different SKU, basically a different model) to an EXISTING Aruba 3810M Backplane Stack...this can be done and it is a supported scenario (in other terms: you can mix different SKUs of the same Aruba Switch Series to form a Backplane Stack).
I suggest you to read the
Backplane Stacking and VSF Best Practices for ArubaOS-Switch thread (there is a very detailed PDF explaining all you need to know).
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Davide Poletto
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