Quite a bit of theorising here, without much research, but I think that you could create a twinborn.
If an Iron Feruchemist were stabbed with an Allomantic Iron spike, wouldn't the person become a Twinborn and also be a Compounder?
If this did work, then how would they be able to compound, because Hemalurgic Allomancers don't burn their Allomantic metal, would they have to be spiked with more Hemalurgic Iron, or would they just be able to ingest more Iron?