Romestan starts as executive vice president on 1 December, effectively as second-in-command to founder and co-owner Charles de La Rochefoucauld at the agency, which employs 50-60 people.
ComCorp has grown predominately by acquisitions since it was founded in 2013, adding French consultancies H&B Communication and Un Coin de Paradis.
Romestan told PRWeek de La Rochefoucauld was now targeting international options for ComCorp, which specialises in a range of disciplines including branding, press relations, crisis management, editorial content and comms strategies for multimedia campaigns.
"It is a business that we would like to multiply by maybe ten [fold] – I think it’s probably between 10 and 15 – in the next three to four years. We want to develop it in the UK but we’re also seriously thinking of Germany, Benelux, Italy and Switzerland," Romestan said.
If the top-end estimate were realised, ComCorp would be 13th in the list of global PR agencies by headcount, roughly level with Brunswick, according to PRWeek's most recent Agency Business Report. ComCorp's revenue has not been disclosed.
The agency – whose clients include Canon, FedEx, HP, Cisco and Lilly - is jointly owned in equal share by CEO de La Rochefoucauld and MD Pierre-Jean Comelli.
Romestan said: "The shareholders have the financial resources to take it further. What I understand from my conversations with the two shareholders is that they are determined to take it further and for that they will have to invest, and I think they are ready to do it.
"Charles is extremely ambitious for this agency. Not only is he ambitious but my understanding is he’s also quite impatient, so he wants to make it a strong European agency that can compete with the big three."
Romestan declined to say which types of agencies would be acquisition targets and if any negotiations are currently under way, but stated: "We’re going to be quite creative and also we would like to take the market a little bit by surprise."
ComCorp is also part of three different global agency networks: WIN, BBN and Ion. Romestan said: "It’s something we want to strengthen in the future. We could also think of taking some stakes in some members of the network, and in the other ways we could also strengthen the partnership with us.
"We are really going to enter a new phase with bold attitudes but open minds, and we will try to make the most of these three international networks as well."
UK 'most exciting' market in Europe
Romestan, whose previously worked in London, said Brexit was not a deterrent to UK acquisitions.
"Despite what is happening, I think the UK is still the most exciting market in Europe. I do think that despite the so-called Brexit, it’s still fine to invest in the UK. So far it has been a bad soap [opera] but it’s not the end of the story and I’m absolutely convinced that common sense will prevail and Great Britain and Brussels will find common ground," he said.
Romestan’s most recent role at WBA was senior vice president, communications and international affairs, a position he had held since the £16bn merger of UK group Alliance Boots and American counterpart Walgreens in 2015.
He said he left the company to spend more time with his family, based in Paris.