You can create new custom templates in several different ways in XenCenter:
VM templates and snapshots that have been exported as XVA files can be imported into XenCenter using Import wizard: on the File menu, click Import, select the XVA file containing the template on the first page of the wizard and then follow the same steps as when importing a VM from XVA; see Import VMs from XVA.
The import progress is displayed on the status bar at the bottom of the XenCenter window and also on the Events view under Notifications. The import process may take some time, depending on the size of the template and the speed and bandwidth of the network connection between XenCenter and the server where you are installing it. When the newly imported template is available, it will appear in the Resources pane as a custom template. The new template has the same configuration properties as the original exported template. To change its configuration properties, use the template's Properties window.