Some features provided by HelpDesk don't apply to Service Desk projects, for example helpdesk-users can not see issue transition buttons even if they have permission to do this.
To solve this problem without giving agent license to regular helpdesk-users it is needed to add HelpDesk Superuser to Service Desk agents. Then all issues transitions will be made by HelpDesk-users if they have permissions to by adding a user to agents for just one moment and then returning user back to helpdesk group.
Please notice that this process requires 1 spare Service Desk agent license.