RTC 6.0.3: How do I merge work item types from different project templates?
Accepted answer
It has to be done manually.
https://jazz.net/forum/questions/137463/importing-a-workitem-type-from-another-project-template
Besides the UI method, you can also try to copy the XML source (only the appropriate parts) and put it into the merged project area.
Also to clarify, usually we do not manipulate the "template" directly. Instead, we customize an exiting project area, and when satisfied, export a template out from it.