Are there any feats, archetypes or similar in Pathfinder that allow you to treat a non-monk weapon as a monk weapon?
Yes, you can
Crusader's Flurry (from Ultimate Combat) is a feat commonly taken for that purpose, it will allow your character to take a weapon, related to your chosen deity, and treat that weapon as a monk weapon for all purposes.
You learned to use your deity’s favored weapon as part of your martial arts form.
Prerequisites: Channel energy class feature, flurry of blows class feature, Weapon Focus with your deity’s favored melee weapon.
Benefit: You can use your deity’s favored weapon as if it were a monk weapon.
Do note, however, that it has heavy requirements to base your build around. It requires both channel energy and flurry of blows. While this could be obtained by multiclassing, you are probably combining two medium BAB classes, which will hurt you heavily and not really justify the combination just to take the feat.
The Sacred Fist warpriest archetype will grant you both requirements, but you still gotta waste a feat on Weapon Focus, and probably another on the weapon proficiency, as the archetype loses both Weapon Focus and proficiency on all simple and martial weapons, substituting their proficiencies to a list similar to the monk's (which is pretty bad). The weapon proficiency has better ways to obtain, such as using a trait or multiclassing into a full BAB class, which is probably a bad idea as it will slow your spellcasting progression.