Sure you can. Just get permission from the contributors first. There are companies (like Digium) which do exactly this, but they go about it in a better way. Also some companies, like EnterpriseDB do that with other peoples' code (again, with their permission). It isn't wrong or underhanded as long as it is transparent, understood, and legal. Joshua can correct me if I am wrong, but I believe that "Mammoth PostgreSQL" was licensed under a proprietary license for a while.
Well the key difference here is, PostgreSQL (what Mammoth Replicator and EnterpriseDB are based off of) is BSD licensed. To answer the specific question, yes Mammoth PostgreSQL was closed source, it is more of an auxilary FOSS project now. Mammoth Replicator is still closed source.
The BSD explicitly allows us to do as we wish with the code, including fork, close source, mutate etc...
LedgerSMB is GPL, which is a whole different legal and ideological mindset, although they are both FOSS.
Sincerely,
Joshua D. Drake
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