If you have a Doom 1 shareware IWAD, it would normally be named doom1.wad.
Doom 1 (registered, 3 or 4 episodes) is doom.wad.
Doom 2 is doom2.wad.

Renaming any of those as doom1.wad will work.

Any PWADs will require the correct IWAD, and additionally will need loaded themselves. Probably via command-line options, but I uninstalled lxdoom some time ago, so I can't readily check.