If you have an open spot on your gourd rack you can remove the gourd arm and gourd and swap out with a feeding tray.
Its not my favorite way as you will be lowering the gourd rack causing the martins to flush. The ideal way would be to construct your own pole, using some wood, and screw the brackets to the wood and insert the feeding tray into the bracket.
For more info on feeding martins locate the Bed & Breakfast product page and scroll down to the "product instruction" then click on the instruction link. There you will find instructions on how to teach your martins to accept supplemental feeding.