The 23-year-old hooker had been named in the Scotland A squad last week, but has now been brought into the senior playing group by the Scotland coaching team.
A product of Dumfries Saints, Hiddleston has scored five tries in 11 matches to date this season, including a double in Saturday's bonus-point victory over Zebre Parma to send the Warriors top of the URC standings.
He joins 19 fellow Warriors in the squad for the upcoming Guinness Six Nations, with Scotland kicking off their campaign in Rome on Saturday 7 February.