There is no easy way to add an existing electrical load to a panel schedule when working on a small renovation project in a building with existing panels.
It would make life so much easier if there was a command similar to the 'SPARE' breaker command that would allow you to insert existing load into a panel without creating "fake" objects. The 'EXISTING LOAD' command can be similar to the 'SPARE' breaker command except it would need to have the ability to choose the load classification, change the wire size, and choose the number of breaker poles.
Currently, when our firm gets a small a renovation project in REVIT, we will export it to CAD, since we know it takes so much to time to create panel schedules with existing loads.