You can do it in the forums. However, the chatroom is on Bike Chat not on Biker Match, yet there are members from both sites sharing it.
And you are able to type in a username manually, without it being registered, if your username already exists (If you didnt log out correctly for example). This would create a data inconsistancy and is therefore not possible.
However, on Biker Match you can click on "browse" and they put the username in the top box. OR, you can go to...http://homepage.bikermatch.co.uk/?matt (replacing Matt with whatever username obviously.