Ok, this is a bit harder than I thought.
I wanted to buy Guild Wars 2 expansions with the current sales, and linking it to my current GW1 account. But the GW2 registration site disallowed me to register a new GW2 account using my GW1 account e-mail address.
I was able to log into https://account.arena.net with my GW1 account details and purchase GW2 and it's expansions from there. But I did not get the option to choose a GW2 display name and was assigned a random one instead.
Is there any way I can get a normal UserName.1234 style display name instead of this monstrosity?