The fastest, easiest, simplest way to do this is with a hex editor. It's improper and downright sloppy, but that's the method I've had the most success with.
The procedure is simple if tedious. Go into Fable Explorer, CREATURE>CREATURE_HERO (1470 if I remember correctly), open the tree view in the entry, click CDefs, up near the top you'll see a link to CAppearanceDef. Click it to open that entry. Extract the raw data. Keep the entry open. Go to graphics.big and identify the ID #s of every animation pertaining to the hero walking, and the bandit walking (FableExplorer w/Search or the "Shadownet" version will be helpful for this). Keeping those ID #s on hand, open the extracted CAppearanceDef file in the hex editor. Replace all instances of the hero animation ID with the corresponding bandit animation ID. Save, close hex editor. Back in FableExplorer, import the edited raw data in the CAppearanceDef entry.
Like many things in Fable modding, it's easier to actually do it than explain it.
Bandits are a premium choice for swapping because they have so many existing animations, bone configurations, different meshes and each mesh comes with different textures. A conversion mod has potential to be done quite proper. Hopefully the mod you're using comes with my mesh swap fix, as this will fix many issues arising from using models other than the Hero's.