Imagine a location, infested with a gene stealer cult. The first Astartes have arrived, but are severely outnumbered. They are slain, over the course of several hours. reinforcements are still many hours away.

The apothecary has gathered as many of the progenoid glands he can from the fallen. The rest are lost. He has them secured, and is making his way back to the airlock in an effort to try and hold out until more of his battle brothers arrive. He fails.

After being slain, the gene stealers now have his precious cargo of glands that will produce gene seed.

What use could the gene stealers make of this?

Could the Patriarch of a cult, and or the Magus, use this raw material to somehow strengthen the brood? Or would this material be essentially useless to them?

By the time the reinforcements arrive, the brood has gone into hiding, and will be trying to do something with their prize...but what?

There is definitely some precedent for space marines destroying gene seed rather than letting it fall into the hands of gene stealers which suggests that if they acquired it they could strengthen the stealers. as to exactly how much stronger it would make them that would be up to the GM. I would say the maximum bonus to give them would be to give them everything described under a space marines gene seed. although quite alot of that would be redundant to a from beyond enemy. possibly just an extra unnatural strength and toughness. that would make for really nasty stealers though, probably want to use them as boss type monsters that there are just a few of in the cult. like a gene stealer human hybrid is weaker than a pure strain, but a genestealer astartes hybrid... well the skies the limit basically