Yes, you can. To do this, go to your profile page on the main site and click on "my logins" at the top right. It will bring up a dialog showing the OpenIDs currently associated with your Stack Exchange account. Click the link at the bottom that says "add more logins." Then, at the bottom of that page, it says
To create a new Stack Exchange account
Click on the link, then fill in your email address and chosen password (and your name if you like), and click the button to submit and that should be it.
Note that this process is separate from any accounts you have on SE sites. To put it another way, Stack Exchange the OpenID provider ("SE-OpenID") is separate from Stack Exchange the Q&A site ("SE-Q&A"). When the page says "To create a new Stack Exchange account" it's talking about creating a new SE-OpenID account. Your SE-Q&A account (which holds your reputation and whatever information you've entered on your profile page on this site) will not be affected, except to have the new SE-OpenID account associated with it. In particular, the name and email address you enter in the form while creating your SE-OpenID account will not be copied to your profile on Physics SE or any other SE site.