Is it possible to "convert" an OpenID account to a regular Stack Exchange account?
There is no such thing as a "regular Stack Exchange account". Stack Exchange recently also became an OpenID provider though.
So to convert your account to Stack Exchange OpenID is as simple as
- register on Stack Exchange OpenID
- change one of your OpenID accounts in your user info (or add the Stack Exchange one if you only have one OpenID account registered ==> you should have 2 registered anyway in case one of your providers doesn't work or gets out of business)