You mean a Feat.
The game had : Might of the Iron Legion - Trained in the traditions of the Iron Legion, you are proficient with Longswords, Shortswords, and Heavy Armor.
But first you need to be proficient with Medium armor, which a Wizard can obtain via a Background or another Feat.
Is this do able by just playing the game, or does it have to be achieve through modding?
EDIT : I did not notice the concext (modding forum) and so misunderstood your question.
What I wrote is all in-game by default. We would tell you if it required a mod.
The D&D rules (5e and prior) make it very very hard for a Wizard to wear Heavy armor, but it can be done, but has a cost. In fact, it is almost impossible in D&D tabletop, but it can be done in Solasta with what I mentioned above.
Generally, experienced D&D players will settle for Medium armor for a Wizard, which is easy to have in Solasta due to a background that allows Medium armor.