I know of 3 off the top of my head but I’m sure there are many more.
Bank5Connect has a 24 month investment CD that allows for add ons. The interest rate is average to below average at just 2.10%.
Mountain America Credit Union allows any of their CD’s to be opened as add on CD’s. Their current interest rates are also average to below average although I think their 60 month 2.3% CD is a decent hedge against lower rates as you can open with as low as $5. It does, however, require at least a $10 monthly deposit.
PSECU CD’s have a non-traditional add on capability in that you must set up re-occurring monthly transfers to add on to the CD on a regular basis. You cannot increase the amount of the re-occurring transfer to the CD but can decrease or stop at anytime. Rates are solidly below average at this time however just a few months ago they had great rates for example 3.25% for a 3 year.
Note: Both credit unions have fairly easy nationwide membership requirements or at least did when I joined.