Support

This is really not possible, nor is it something that is going to happen in the future. The crux of the problem is user management. This is also why no other extension, self-hosted application, or SaaS offers anything even remotely close to this, but you're welcome to look around and find something you think might work.

Based on our experience, what you want is impossible and hits a brick wall very soon. Each site has its own users (therefore user IDs, which is what the tickets use to determine who submitted them and who is answering to them), user groups (therefore user group IDs which control permissions, including whether a user can submit a ticket in a category and/or view tickets in that category), and view access levels (therefore access level IDs which further controls whether users can view tickets in a category).

You want to make it so that when one user is logged into site A ("their" site) to also be logged into site M ("main" site). Not only would you need a Single Sign-On solution across all these sites, you'd also need to somehow translate all the user, user group, and access level IDs across all sites and not have duplicate usernames across all sites.

There are easier ways to drive yourself nuts.

The best thing you can do is what everyone does (for a very darned good reason): have a dedicated site as your helpdesk, and have user accounts just for the helpdesk. If that's not something you're interested in, I believe you might be in need of a CRM software, not a helpdesk.