If you want to pause on a single system for a while, press and hold the FUNC knob for a couple of seconds (this latches the "FUNC" function and holds on the current system). There is no way to hold on more than one system using this method.
If there are systems you regularly want to isolate from the rest of scanning, you could create versions of them assigned to startup key 1 and put the rest of the systems (including duplicates of those) on startup key 2. Power up while holding down "1" and all systems except for those would be locked out. Power up while holding down "2" and those two will be locked out and the rest will be unlocked.