Yes, you can do this but I would recommend starting with a new mail profile;
http://www.howto-outlook.com/faq/newprofile.htm
After this, instead of setting your original pst-file as the default mail
delivery location, only connect to it by File-> Open-> Outlook Data File...
After connecting you can move your emails from your orignal pst-file to the
newly created Inbox, Outbox, Contacts, etc... folder. This way you'll be
using the new UNICODE pst-file format instead of the old ANSI format.
Office 2002 applications cannot integrate with Outlook 2007 so some
integration functionality (like Word's mail merge) will no longer work.