If you have a domain and an exisiting website, when you register for Digital Towpath you will be assigned a sub-domain of digitaltowpath.org to use as a development site. You can build your new DTP site at that location and, when you're ready, we can move it to your current domain, replacing your old site with the new DTP site.
Note, though, that during the development process, until your DTP site is live at your domain, you will not be able to use DTP Email Service.