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)
To provide further details to fretje's helpful answer, in order to choose your Stack Exchange OpenID in the "add login" page, enter:
in the input box labeled for manually entering your OpenID.