Use standard CREATE TABLE SQL DDL commands specifying:

The full Unity Catalog table name: catalog.schema.table

The exact column schema to match the external Delta table schema

The external LOCATION of the existing Delta table

The USING DELTA clause to specify data format

This will register an external table in Unity Catalog that points to the existing Delta files without moving or modifying data.

The table properties must match the existing Delta table properties exactly.
If Unity Catalog/Databricks adds extra default properties such as collation, and they differ from the Delta table's _delta_log properties, you may face property mismatch errors

To avoid conflicts with automatic property additions (like collation), explicitly specify all existing properties in TBLPROPERTIES

to onboard multiple tables efficiently:
Create a script or notebook to:

Read the schemas and properties of all existing external Delta tables.

Generate the SQL CREATE TABLE statements with schema, location, and exact properties.

Execute those SQL statements in batch to register all external tables in Unity Catalog.