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So the lore makes it clear that each Chapter must use their own stock of geneseed. If the stock is contaminated, robbed, or destroyed, then it's gameover for them. Even the Emperor's Children before the discovery of Fulgrim had this problem. However, there are some exceptions:

+ Chaos Space Marines will plunder geneseed from other Chaos Space Marine or Loyalist Space Marine Chapters all the time, regardless of Chapter/Legion. Honsou was created from Imperial Fist geneseed and I don't see him suffering from any defect.

+ After the Baal Campaign, the Blood Angels ordered their Successor to give them some recruits, and likely the geneseed these recruits already have.

So it is possible for one Chapter to convert the geneseed of another Chapter so that it is fit for its own use? If a Chapter is destroyed/disallowed from recruiting, would another Chapter be allowed to use the excess geneseed, instead of locking them away? If not, would Successors of the same Legion be able to exchange geneseed should one Chapter falls below strength and another having too many?

Using another Chapter's gene-seed instead of your own is essentially just wiping out your gene-line.
More closely related successors would have fewer differences, but there is still likely to be distinctions having arisen over time.

That's not what the Imperial Fists say.