See letter from General Aviation and Commercial Division, Flight Standards Service dated 24 April 2017:

http://indocap.com/FAA/FAA-AFS800_Palmer1.pdf

This is the solution for certificated pilots who wish to add a Glider rating but cannot solo because they have no current Flight Review and therefor cannot meet the requirements for a Private Glider add-on.

Now simply get a Sport Pilot Glider add-on which substitutes for a Flight Review. Once that is done, the pilot can solo which will allow him to meet all of the requirements for the Private Glider add-on. Also, once he has Sport Pilot Glider privileges, all flight time, including instruction time, will be PIC time which makes it easy for him to skip the Private and go straight to the Commercial Glider add-on.

Stephen