It looks like it's not possible to loopback unit procedures. Per Sepasoft documentation: "The workflow validation does not allow Unit Procedures and Operations to exist inside of loopbacks".
Will there be any future support on this? This is possible and allowed to do in FTBatch. I'm considering migrating from FTBatch to Sepasoft Batch and I'd like to keep the same functionality I have currently especially to reduce any additional validation efforts (in a regulated environment).