Is there a way to reserve Slots for manual Backups?

Well the title sums it up.

But let me explain.
Our Current setup has 4-Slots for simultaneous backups. Those 4 Slots are blocked the whole day by scheduled backups (one machine finishes - another one starts).

The thing is that our users are allowed to start file-backups manually (this wasn’t a requirement until the Project was finished and launched… but you know how it works, requirements grow over time…).
But as the 4-Slots are always used, the manual backups don’t start on time.

Does anyone of you guys know a way or a workaround to solve this?
like 2 extra exclusive slots for manually triggered backups?

Thank you guys
Best Regards