You can add the leadership to each field in the append/update or make table query you use for your Mail Merge tables. If the field is only pulled once at the top of the page for example, it won't matter that it is returned in each record since it won't be pulled on the form letter. In fact, it will be pulled once for each letter, so it will be used once for each letter. So that should work fine.
So if you add the leadership field to the mail merge tables, and then modify your form letters slightly, this should work. I don't think you need to do a second data source. Once you modify the table and open your Word mail merge doc, the field should show up in your current data source. Use it as often as you need it.