Is there any way to add custom attributes in Keycloak via REST API?

I guess you mean adding user attributes to the admin console by extending the theme - https://www.keycloak.org/docs/3.1/server_development/topics/custom-attributes.html Since that configures the admin console itself it does involve some configuration of files loaded by the keycloak app for a custom theme so I don't think the REST API alone will be enough.

As @Xtreme Biker points out, anything you can do via clicks in the admin console you can do via the REST API as the console uses that API. You can perform the relevant actions in the admin console and check the network tab in the browser console to see what the REST calls are (note you may need to tell your browser not to clear the log between page loads). So if you can do it just with clicks in the browser then the REST API is enough. If you also need to modify configuration files then you'll need to do that part outside of the REST API.

Here is an example

curl --location --request POST 'http://yourKeyclaokSSO.com/auth/admin/realms/YOUR-REALM/users' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer <INSERT TOKEN HERE>' \
--data-raw '{"firstName":"James","lastName":"West", "email":"jw@test.com", "username":"james.west", "attributes": {"SomeId":"123"}}'

More documentation: https://www.keycloak.org/docs-api/11.0/rest-api/index.html#_userrepresentation