As far as I can tell, although possible, there would have been absolutely no reason for Ruin* to give Marsh a spike for Atium Feruchemy, especially since he was planning on destroying the world. On the other hand, Brandon said that Marsh can Compound Atium, and therefore he can.
Another thing to remember is that if Sazed can give Spook full Mistborn status, he can probably give Marsh full Feruchemist status, if he so chose.
*theoretically, TLR could have had the spike placed, but he wouldn't have wanted any other pseudo-immortals competing with him, so he almost certainly wouldn't have give Marsh the spike either.