You can still do it in a 2 level priority if all your present interrupts are the same priority.
--
what if programmer wants 2 priority levels
Then you need a 4 level chip to do it. This is a neat trick to use when you can, if you can't you need other means.

Does your RTOS allow a task to say "need no time now".

Another possibility, if your RTOS allow it, would be to do, what I suggest done in a software interrupt, in a high priority non-preemptable task.

Erik