Paster - this week appointed to the newly created role of EV-P PR and public affairs at WPP - said the marketing services group's PR interests would continue operating autonomously, despite previous speculation that some brands would merge.
He steps down as H&K's chairman and CEO after nearly nine years, handing over H&K's top global post to president Paul Taaffe, who has been groomed for the role as part of a succession plan.
At WPP, Paster will work closely with CEO Sir Martin Sorrell with a remit to oversee the group's PR businesses, which include H&K, Ogilvy PR Worldwide, Burson-Marsteller, Finsbury and Buchanan Communications.
New York-based Paster said: 'The parent company has been nurturing, building and helping companies grow, but at this stage it needs someone to take responsibility for the PR groups.'
Paster added: 'The PR category is going through its second difficult year in a row and we need to stop decline and build up from there.'
Formerly H&K's UK chief executive, Taaffe is appointed to global chairman and CEO after four years as president - a post that has now been scrapped.