Realm character change or transfer

Hi, it's been quite a few years since I played Warmane, and now I want to start again, but I see that my character is in Icecrown. I want to know if it's possible to pay for a service to transfer it to Onyxia so I can play with my friends. I don't know why their characters are on a different server when we used to play together

No, it isn't possible.
Also, Onyxia released not even two and half years ago. You couldn't possibly be absent "a few years" and have played there anyway.