Who says you can't??? All you have to do is to add an appropriate training syllabus for the mentioned types. Being an FTO you should have a Quality Assurance Programm so there really is no reason to apply for a TRTO.
You are even able to do type conversions for licence holders simply by adding an appropriate training programm to your AOC. The reason for that again: the required quality assurance programm which already is part of your AOC.