Doctor Who Magazine reports that Laurence Belcher and Leo Bill will join the previously cast Michael Gambon, Katherine Jenkins and Danny Horn.
Child actor Belcher previously appeared in the BBC's Little Dorrit, while Bill starred in Steven Moffat's Jekyll in 2007 and most recently featured in Tim Burton's film adaptation of Alice In Wonderland.
Micah Balfour and Pooky Quesnel will also feature in the festive episode. Balfour is known for his role as PC Benjamin Gayle on The Bill, while Quesnel starred in the second series of BBC drama Criminal Justice and recently appeared in EastEnders as Rachel Branning.
Filming on the special is now complete and production on Matt Smith's second full series as The Doctor is expected to begin shortly.
The currently untitled instalment will air on BBC One at Christmas.