You're talking about internaly plugging an aluminum male heater hose nipple? Yes you'd want to NPT tap it if you do & not sure if there's enough wall thickness to work with (there might be just never looked at it that way). Much easier would be to get a black neoprene end cap from the HELP section at your parts store along with some gasket cement and a hose clamp. You could paint it engine color to try & hide your handiwork. Functionally it'd be fine