Allen played CIA deputy director Pamela Landy in The Bourne Supremacy and Bourne Ultimatum, while Finney featured in the latter as Albert Hirsch, a doctor in the Treadstone programme that trained Jason Bourne.
The pair will join Edward Norton, Oscar Isaac and Rachel Weisz in the supporting cast for the spy thriller.
Michael Clayton director Tony Gilroy's new Bourne will jettison Matt Damon's character and centre on one of his fellow Treadstone operatives, to be played by Jeremy Renner (Mission: Impossible - Ghost Protocol).
Damon last year voiced his support for the Bourne series's change in direction, but suggested that he and director Paul Greengrass may return to the franchise at some point.
"I think it's a smart thing to do. It doesn't interfere with Paul and me doing a movie," he said of Gilroy's Bourne Legacy plot. "Tony doesn't want to hurt the Bourne story. So as I understand it, Jason Bourne is alive in the world that Tony's going to create. I don't appear in it, but it's very much that world."
The Bourne Legacy will begin production this year for release in August 2012.
Watch the trailer for The Bourne Ultimatum below: