UPDATE December 10th, 2020, please also see: https://www.odoo.com/documentation/user/14.0/db_management/hosting_changes.html 

Generally it is.  Please try these steps.

1. Plan a time to do this after the weekly update of Odoo.sh, and update your On Premise Enterprise server to the lastest daily build of Odoo.  This is important.  Odoo.sh is updated at the start of each week and the code on your Enterprise Server needs to be similarly updated.  You need to make sure the same custom code repositories are available on your Odoo.sh database as are available on your enterprise server.  This means uploading the modules from your Enterprise machine to the Odoo.sh GITHUB repository. Go ahead and create an Odoo.sh Project as normal, and ideally you test this process with a staging branch.

2. Take a backup from your Enterprise database:

3. Activate the branch (STAGING or PRODUCTION) that you want to restore your Enterprise database to.  Make sure this branch also has the full source code of all Custom and/or Community modules installed in that database. From the BACKUPS tab, you can import the database backup you made:

4. Find the BASE module and upgrade it: