His practice has been asked to look at remodelling the public space beneath the Grade II-listed London tower by its new owner, the Targetfollow Group.
Farrell will also investigate the impact of the multi-billion Crossrail project on the 117m-tall block.
Meanwhile, the owners have chosen Rolfe Judd Architects to carry out a major internal refurbishment of the 32-storey skyscraper, which was built between 1962 and 1964.
The appointments form part of a major overhaul of the centre, which is to include the opening of a restaurant and private members' club on the top two floors.
Ian Fox, a director with the Targetfollow Group, said: 'We are pushing ahead with exciting plans to completely revitalise this outstanding landmark and transform it into an unrivalled business destination.'
He added: 'It enjoys one of the most central locations in the capital and will greatly benefit from plans to improve the Tottenham Court Road tube station and create a Crossrail station there.'