Adding actors to a room
Don't even bother; visit this link: 
This tutorial shows you how to add actors and groups to a room in your ROM with a hex editor, then modify it with UoT.
(Note: Everything is in hexadecimals, and you should read the "IMPORTANT NOTES" section) Go to the start of your .zmap file, and look for 0B, beside it is the number of groups Example: 0B04 says that there's 4 groups in the map. Then modify that to your own wishes.
After you've done that, look for 01, besides that is the number of actors. Example: 0105 says theres 5 actors
Modify that to whatever number you wish, and save the rom and go to UoT and start fixing. :)
IMPORTANT NOTES: -You can also modify doors(exits?) by adding to the "0E" in your scene's header with the same procedure -This is written basically for help with porting (You have to remove actors, groups and doors while porting). -Though it's written for porting, it also works for modding purposes -When Adding actors (And groups?) you have to add actor information to the end of the .zmap, tutorial written by Jisaan can be found here: 
Hope it helps! -Jani