Helpdesk plugin has a 3 roles: Admin, Helpdesk Manager, Helpdesk client.
Standard WordPress methods allow you to assign the following roles to a user: Administrator, Editor, Author, and Contributor.
This is not possible in WordPress currently to assign 2 roles to one user at the same time. You can just сhanging user roles. However, you can use some different plugins, which help edit user roles and giving more control of WordPress’s role mapping.
For example, you can solve it the following way:
1. Find plugins that allow edit user roles, for example “User Role Editor”, and install it.
2. On the users tab, find the user whose role you want to change. And choose Capabilities.
3. Than assign an additional role to user and “Update”.
4. Then go to the tickets tab and check. The “Assigned to” select box should contain several items.