Here is my experience… I took a custom database from Heroku. And migrated it to Xano. Then set up Adalo as a front end using an external collection for the back end. That way I can use the same back end for my current desktop web AND the new Adalo app.
It should work the same for Airtable, Backendless, or any other external database.
**FYI - I was not able to migrate the passwords from Heroku to Xano.