As far as I know, there is no out of the box way to do that. I have also not seen any customers who have done that.
Regards,
Alison Sanchez
TeamSite Support
What you can do is use the callout feature (on a readonly textarea) to pop open a custom window that would contain a custom version of TinyMCE. I have seen this done where the requirement called for using TinyMCE 4 in earlier versions of TeamSite. I wouldn't recommend trying to load the OOTB version of TinyMCE in a custom window though.