Code in Fey AI to transport items from inventory A to B.

Probably set the mob's data, a BlockPos for input & output through a wand, right clicking in the world will set the data to the wand, clicking on the Fey will transfer that data.

Then in mob tick goes the ai, and if the input has an item, and output is not full, then grab an item stack from input, and put it into the output.
Else just wait near the input.