Ah, the confusing terminology. What I think you're describing is support 'units' and there are tools to manage those
The support 'squads' are different. The game describes them sort of implicitly, and you can see how many you have and how many are required at a given unit/HQ/airbase X as "Support/Need" numbers listed on the right side of the unit windows.
What I don't understand is what you can do to unit/HQ/airbase X to change things if they are short of support squads there.
Though it's not explicitly mentioned, I'm guessing support squads get made up from whatever men aren't used for other stuff (refitting, replacements, new support units, etc.) so that the extras get turned into support squads and handed out by algorithm. So if things calmed down on the front so refitting and replacing slowed, and you weren't creating new units to soak up the manpower, there'd be more men to make support squads from for the entire army, and eventually unit/HQ/airbase X would get some more.
Or, hopefully I missed something and there's a more specific way to increase support squads where needed.