Buck, who is expected to be a spokesman for the VP nominee when a selection is made sometime this month, will be working with Kevin Sheridan, a former Republican National Committee spokesman who will head up the communications shop in the vice presidential pick's office, and Romney national press secretary Andrea Saul.
He took leave from the speaker's office and started the job this week in Boston.
To date, Sens. Rob Portman (R-Ohio), John Thune (R-S.D.), and Marco Rubio (R-Fla.) have made the rumored shortlist along with Rep. Paul Ryan (R-Wis.).
"Brendan will be a great addition to the campaign. We’re looking forward to him applying his quick wit and deep understanding of President Obama’s record of broken promises to our effort. We’re lucky to have him," said Sheridan.
— This story was updated at 12:03 p.m.