Aux units can't be upgraded in any way, but if you change this unit to core then it should be possible to add transports later. 76mm_m1939_Germany supports most types of German transports. I'd add a horse-transport in the editor as cheapest and common artillery tractor "for immediate use against the enemy". It's the same story like in the Winter War scenarios where towed guns without transport are almost useless when gifted during scenarios.
I'd always change gifted units to core, so even if you have no use for these after the scenario, you can sell them as bounty for a few bucks.
Historically, it often took quite a long while until equipment was made compatible, like adding own radios, sights and most importantly a manual in own language to operate and maintain the gear, or even rechambering the guns like with the Pak 36(r):
What we often enjoy in games is rather the hollywood-style to jump into half-damaged tanks and immediately shoot the crap out of enemies to receive a medal of honor.