[Ryan F]

There is not a feature to do this but it owes to the fact that PF2 just simply doesn't work this way.

If you're looking to make a customized NPC with abilities comparable to that of the various Classes, you can do that via Added Abilities and the detailed NPC building options you can unlock with the Gamemastery Guide, but in terms of just mechanically making an actual NPC using the NPC rules with Class levels, that is simply just not part of how the game rules function.

Should you wish to make NPCs with Class levels you should instead just build them as a PC and then afterward you can do a bit of manual cross-referencing to check their stats versus other NPCs of the targeted level in order to use various tweaks and the use of Adjustments to get the stats where you want them to be.