can you run bathroom exhaust fan to central air return?

Typically the bathroom exhausts to the outside to remove the moisture in the air. I would not use the return air duct because it does not directly vent to the outside and you could increase your moisture content in your home. This is obviously problematic due to increased spore/bacterial, allergens, etc.

I don't think you've thought this entirely through. What happens when someone drops a bomb in the bathroom and all that stink gets piped into your central a/c return? Technically speaking, I don't see any reason why you couldn't do this, but the real question is do you actually want to do this.

Just go by a air purifier and some candles and put it in there. If there is a lot of moisture build-up, they make products that catch moisture and puts it in a cup/bowl to be empty.

no don't do that. Run it threw the wall to out side. stink is not the prob humidity from showers is.