less of an issue more of a question

[Sparky120]

How does one add already existing mobs to the treadmill

[onegentig]

By pushing them on the treadmill, using a leash or by luring them with a food item.

Also note that in some modpacks, like in Sky Factory 4, the Shell Constructor instead connects to a regular power grid.

[iChun]

Codewise it's a matter of mapping the entity class and the power output they're meant to give as seen here https://github.com/iChun/Sync/blob/7419e7e91b6501745086caaa15e8204b413d44ff/src/main/java/me/ichun/mods/sync/common/Sync.java#L94-L95