Selections can never include units from multiple systems (control groups can, but when you select that control group, it only actually selects the units in your current system in that control group). There is some limited ability to order units across systems from the galaxy map, but that is for movement, such as moving all scouts to one place. You use the magnifying glass on the left, below the P1-P9 buttons. So the short version is you can't.
The long version depends what you are trying to do. I can assume you want to pause or unpause them all. In that case the fastest way is to use the quick find buttons in the bottom left. Click on the one that looks like a space dock, select Dock(100) and it will list all your Space Docks. Shift click on each to pause it. When paused, it will disappear from this list. When you want to unpause them, click on the red battery (which is a list of all paused units) and repeat the process to unpause them. If you click instead of shift-click you will switch to that unit's system.