TMP form limitation

I am updating a TMP form and I want to create a new 'People' field to capture a user field in a form (different to the submitter, assigned or watcher) but am noticing that there is a limitation on the 'People' field - when I add it in (in the field building phase) I do not see it in the form building stage.  Is there a way to get this limitation removed or is there any advice on a work around for being able to tag someone in the form?

Thanks in advance!

Shannon