Hi Jason, that should certainly be doable. Why haven't you had any luck with the "Fill an area of the document" option? If you're not seeing the fill area listed there then you should be able to go back to your merge form, add a fill area where you want it, give it a nice name, and it should be available in the workflow for you.
If you're not seeing the fill area listed in the workflow, then check the trigger conditions of the workflow to ensure that it's set to use the merge form (let me know if you need help with that).