CiviCRM Community Forums (archive)

You can include a profile in your online contribution / membership signup page (generally used for "self-service" membership signup and renewal).

If you're referring the the "back-office" New Membership form (from the contact Membership tab) - you cannot currently do this. However, the upcoming 2.3 release includes workflows for "new membership for new contact" which allow you to customize a profile with the contact fields you want to collect when adding a member. You can try this out on the 2.3 sandbox:

http://sandbox.civicrm.org/

After logging in (demo  demo), select "Membership" from the "create new" shortcuts drop down menu on the left side of the screen.