Can I add a field (like a mail merge field) to a text box?

I’m using Writer, and am trying to plop a few fields into a text box that is within a header. I cannot seem to add fields to the text box, nor to text boxes outside the header. It seems as though the only way I can get fields onto the page is to put them in a regular text area (that is, not a box). Is there any way I can add a field to a text box?