Thank you so much for the detailed reply.
I can’t believe it was so easy. (I hope I haven’t missed anything during a quick test, but it seemed to work perfectly on my dedicated server)
I just added the S+ folder structure >Mods>StructuresPlusMod>Crafting>Smithy
Copied the original smithy into this folder and renamed it to BP_SmithyPlus
Then in my mods PrimalGameData blueprint, I just added it to the ‘Additional Structure Engrams’ section.