Yes, you can look up any scopes using scopes?scope_strings, as long as the scopes are advertised or the owning client is public.
To create the dependency, you would then provide this in the dependent scopes field when you create your scope. For example, the create scope payload would be something like (using the ID for urn:globus:auth:scope:transfer.api.globus.org:all = 80fa5a88-ae26-4db7-be3a-c5f4cf4ac8d2):
'name' : 'ExampleScope',
'description' : 'Example dependency',
'scope_suffix' : 'example_scope',
Please let us know if we can help with any more information. Thanks for highlighting this by the way, we'll add more documentation around this call.