What are the regs/guidelines for offering an add-on certificate of deposit to our products?

Do you mean what are the regs and rules regarding developing and offering a new product?

Regulation DD is going to apply in this case, since it's a certificate of deposit product. You'll need to research the reg carefully to be sure that your product is structured and disclosed in accordance with DD. You'll also need to compare any advertisement of the product to the rules in DD.

There are some other rules and regs that might possibly apply, depending on the particulars of the product.

P.S. Buster, remember to only post a question in one forum. Posting in more than one at a time is confusing and frustrating to everyone - those trying to answer your question, and those trying to read the answers!

If "add-on" refers to an ability to make subsequent deposits, you'll need to be able to track the date of the most recent deposit for managing any early withdrawal penalties. Of course, Regulation D's definition of time deposit is something you'll need to adhere to.

I don't "do" deposit compliance but had this question asked. When the CD is in its grace period can you add addtional funds to the CD?

The add-on CD usually means during the term of the CD. It could be done under this clause during this grace period.

If what you have isn't an add-on CD (contractually allowing the additions and often in some predefined increment) it could be done at this time as well. The grace period allows the customer to end the current contract, add new money to a new one, and create a new CD.