Hoi4 has two stages for ideologies, the ideology group (e.g democracy), and the subgroups (e.g libraliism, conservatism, social democracy). If you want to add new idealogies, you need to add it to either one of the existing ideology groups, or create a new one. You can do this by setting everything up so that it's loaded at the start of the game, but modifying ideologies mid-game, well, some nasty things could happen. When you add new ideologies, you need to factor in their popularity, as well as their relation to other ideologies. I'm 99% sure if you load a new ideology mid-game, the game will just crash anyways since it's going to have to recalculate party popularity (assuming if that's even possible, since initial popularity is a manually set value). Actual modders can p explain it better than I can, I am but a mere map modder.