© PA Images / Dennis Van Tine
The OC star previously played Cindy, the roommate of the eponymous mother, in two episodes of the CBS sitcom.
Thomas told TV Line: "She's still the roommate of the mother from the title of the series. We didn't establish her lightly."
He added that the Cindy character will play a vital role in Ted (Josh Radnor) eventually meeting his future wife.
"We definitely intend to show how she's part of Ted's link to finding the mother," said Thomas.
Bilson will next be seen in forthcoming CW comedy-drama Hart of Dixie.
How I Met Your Mother will return to CBS on Mondays at 8/7c in the fall.
> Conan O'Brien wins HIMYM walk-on role
Watch an extended trailer for Rachel Bilson's Hart of Dixie below:
