Sullivan, nearing the end of a three-month loan spell at Cardiff, is one squad away from being the first genuine league player to play international rugby union. Henry will name his final 22-man squad on 26 January for the Murrayfield game, which takes place on 6 February.
If selected, the 30-year-old Sullivan will be available to play in the championship and is one of five players Henry has added to the 22 on duty against South Africa and Argentina in November. "It is the first step in achieving my ambition of playing for Wales, but until I am named in a 22 I still feel it is a long way off," Sullivan said. "This is a great opportunity for me to be involved at international level and it is down to all the help I have had at Cardiff."
London Irish prop Peter Rogers, West Hartlepool wing Steve John, Richmond centre Allan Bateman and Cardiff prop David Young are the others added to the squad.
Henry will meet up with the players tomorrow when he takes a training session at RAF St Athan, near Cardiff.