Ospreys back row forward or lock Thomas (shoulder) and Scarlets wing Jones (ankle) were both left out of Jenkins' original squad but have made significant recoveries from injury to now warrant inclusion.
Jones, however, remains an injury doubt for this weekend and, with fellow wing Shane Williams (hip) also in a race against time to prove his fitness, Wales under 21 wings Aled Brew (Dragons) and Jamie Roberts (Blues) have been invited to train with the squad in an unofficial capacity.
"Mark and Jonathan both missed out initially because of the recent problems they have had with injury and they make a welcome return to the national set-up," said Jenkins.
"We have also called on Aled and Jamie to come and train with us in the light of some acute injury problems we have in the backs.
"They will be with us on a day-by-day basis and it is a real opportunity for them both as youngsters to get used to the international environment.
"We have taken a conscious decision to look forward and tap into the quality we have in age-grade rugby. Both are talented individuals and, if our injury problems persist, we are confident that if called upon either player could do a job for us."
Jamie Roberts, CARDIFF BLUES (Wing), DoB: 4/11/86, Height: 6' 1", Weight: 17st 0lb. International record: Uncapped
Aled Brew, NEWPORT GWENT DRAGONS (Wing), DoB: 9/8/86, Height: 6' 1", Weight: 14st 9lb. International record: Uncapped