Is it possible to create a workflow in SharePoint designer and then import it into a SharePoint site? Does the workflow need to be a resusable workflow?
Wht do you need it to do? List level workflows aren't portable; site level workflows aren't much better. Reusuable workflows are designed with portability in mind. They can be attached to content types on lists / libraries too under thier respective workflow configuration screens.
If you can give us a little more information on what you're after that'd be great.