One thing to note about Support Units, is that they are always by default on refit with no way of turning it off. Kinda sucks but when you think of playing around with refit on 100s and 100s of Support Units, it's probably best and simplest.
The effect this has, however, is that it can sometimes make it difficult to direct replacements via on-map unit refit. I've noticed that Russian artillery can really suffer from this, as there can be a lot of Rifle Divisions and artillery support units sucking up artillery replacements, causing on-map Artillery Divisions to fill up slowly. The one aspect of player control that can affect this is TOE %. However, this works best at directing replacements AWAY from support units (by setting that support unit type to 50% TOE) and into on-map units, not the other way around. The only thing you can do to give a small priority to particular support units, would be to reduce the majority of that unit type to 50% TOE, and leave your priority support units at 100%.
In the case of STUGs, because other on-map units will use that equipment you'd also have to reduce the TOE % of those units to give better priority to particular support units, but it's generally best not to focus replacements to support units by sacrificing on-map unit strength.