That's what I've been doing - keeping the shells from resupply/spares formations around to use for that purpose. I standardized on 30K as my regimental transport size, with formation sizes of 1K for HQ/construction, 10K for infantry, 10K for tanks, 5K for artillery, and 2x 2K spares slots. The initial builds take a long time but logistics and reinforcement units build pretty quickly.
But I think you could create a null unit (new formation template, don't add any units) then use "Instant Build" to make a new shell. Or if you want to be safe you could add say one logistics infantry and create the new unit for a cost of 0.2 points.