Hi John,
Yes, you can add a "Comment" Field in your EFT File.
Here are the steps:
1. You need to add a Comment field in your EFT File Format >> Select Line Type = Detail >> In my example I position it at the very end...field 11, then set your length (max 30) and description.

2. On field 11, click on show button on the right to display table mappings.
Look for Table Name <PM Paid Transaction History File > and Field Name < Transaction Description. <SAVE>

3. In Edit Payables Check Comment, enter your description.

Note: You may need to go to Purchasing >> Transaction >> Edit Check -- if you have more than one record

4. Generate EFT like you normally do.

Here you go... EFT File with Comment.

Hope this helps!
Regards,
Cecile Dinh