It kind of depends what "large lathe" really means. If it has a center-to-center length of 80" or so, it would be long enough to drill a half scale Napoleon. However, you will need to drill a 31.5" deep hole for the bore, which is a non-trivial process. If you have the tooling for that, I would be sorely tempted to buy a 3' piece of 6" round 1018 or 1020 steel and turn a whole new barrel. More expensive but you would have an all steel barrel which wouldn't need a liner. The shape of a Napoleon is not complicated; only one long taper, one shallow radius, a couple of small radii on the muzzle and then the cascable.
Drilling out the old barrel would be an interesting exercise due to the asymmetrical distribution of metal. The hole would have to be bored, and getting the chips out of a 32" deep hole virtually demands oil being pumped down to the area of the cutting. I would recommend boring to 3" because that would get rid of all of the old bore. But unless the liner is fused to the casting, there would be a significant risk of the liner remnant coming out. Also, the casting would be only .25" thick in the small area behind the muzzle.
Filling the bore with metal before drilling is likely to be impractical. The whole barrel would need to be heated to at least 600º F and unless the filler fused with the liner, it wouldn't really help.