Can an entire process template be imported to an existing team project

You can upload entire template using Admin UI or use ProcessTemplateManager.exe, or IProcessTemplates interface method as in http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=454833&SiteID=1 . When you say "into an existing project", do you mean updating the current types and data with new template? You can update the workitem types individually by running witimport, but not overwrite existing project with new template data.

If I wished to do such a wholesale replacement I think the best approach would be to just branch using the new template.

I would like to clarify what a template means to a project after a project is created. A project is simply an instantation of a template and is free to be modified later on. After a project is created, all parts of it can be modified (work item types, documents, reports, CSS hierarchy, any 3rd party elements, etc) to an extent that it no longer resembles the template it was created from. That is why no association between the template and the project is kept in the project properties. Any effort to relate back to the original template would be futile for this reason. Think of a template as a starting point, not a fully defined project by itself.

So as Yogita suggested, your best option is to create a new project from your new/modified template and migrate in the work items, documents, source control, etc as needed.

-Ravi