If you add a unique key on a table. And then add this key, the Attributes composing this key pass automatically in Not Null.

Add this key, in the entity properties, add keys and select attributes. Press OK to create the key, all attributes moves automatically as Not Null.

When tested directly in SQL Server, it is possible to add table with attributes null in a key.

Unfortunately, there is no way in Toad Data Modeler to set the attribute to be nullable. The checkbox is grayed out that's why user cannot modify this setting.


STATUS:

Waiting for fix in a future release Toad for Oracle.

In majority of cases having nullable column in key is not a good practice.