ZWADCONV

From ZDoom Wiki

ZWADCONV is a utility to convert Doom format maps into Hexen format maps. It can be found in the ZDoom editing utilities package found here.

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from zwadconv.txt

Usage: zwadconv <source.wad> <output.wad>

This program converts every map in the source.wad into the enhanced format supported by ZDoom. This is essentially what the game does when it loads a normal Doom map. However, ZDoom is free to use certain tricks which this program can't, so it's possible that some maps will not convert successfully. This is an external utility, so it may not be possible to convert every map perfectly, as well. The file convlog.txt will contain a copy of every message printed to the screen during the conversion and should be checked if something went wrong.