How to add a table to the available table fields in the e-mail template form

Hi,

We need to add a table that is not available by default in the e-mail template form (found in /Organization administration/Setup/E-mail templates / E-mail message button).  The tables and fields are in the Insert / Fields menu on that form.

An ISV of ours has added one of their custom tables to that form, but I can't figure out how they did it.  Has anyone done this?  If so, please share the steps required.

Thanks in advance.

djz