It isn't possible to associate a custom Search Resource / Resource Configuration / Similarity Definition with Agent Assist in Agent Workspace.
A custom Search Context can be created and can be configured to have more/less 'Additional Resource Configurations'. However, it isn't possible to create a Search Resource or a Resource Configuration record or modify the existing records.
Also, this is leading to be unable to add a custom Similarity solution of Predictive Intelligence to Agent Assist.
Actual behavior: The New button isn't visible for neither of the lists
Expected behavior: The New button is visible to create a custom resource (to add a custom Similarity solution)
This is expected behavior and working by design in the current releases.