Did some research and there doesn't seem to be a way to add additional categories to the TA function. Would the Citations & Bibliography tool, using tags for categories, be a better fit for your purpose? I know that sounds like a lot of rework, but if you are doing a big research project, you might need more documentation about your sources than provided by the table of authorities tools. Sadly, the TA field is internal to the document whereas the Bibliography manager maintains source data in a separate XML file and there is no easy way to transfer info from one tool to the other. BUT... you can generate a table of authorities for the citations you've already created, build your Biblio source references, then use the Find > More > Special > Field tool to find every instance in which you've created a TA citation and replace with Biblio citations.
Sorry I don't have the easy answer.
