I noticed there was an option to login using ones Steam account, I tried it and was presented with the AVSIM "Create New Account" page. When I entered my login name I received a message saying that username has already been taken.

Do I understand your question correctly when I assume you want to associate your steam id with your current avsim account?

Why sign in through Steam?

By signing in through Steam,

forum.avsim.net

will be able to identify you within the Steam Community and retrieve your public gameplay information (such as stats and achievements).

Steam will not reveal your username or password to this site, rather a unique numeric identifier will be shared. You should never enter your Steam login credentials into a third party website!

 

Yes but I do not want to have 2 login ID's for AVSIM

Unfortunately it's not possible to do once you're already registered. It is an administrative nightmare so to speak.