In the list of issues in JIRA Portfolio, I can manually assign possible team members to a task by checking the check mark next to their name.
See this screenshot from your documentation:

In JIRA we already use a custom field with multiple users that we call "Assignee options". We fill that field with the users that are able to take on the issue. We use that field to create Todo lists etc. on people's dashboards.

I would like the contents of such a field to be used for "manually" selecting possible assignees in Portfolio.

Let me know if my request is not clear or if I need to explain it further.