Woocommerce: set product as “ON SALE” without sentting a new price


I need to set (programatically, from a custom plugin) some products as “ON SALE” but these products will only have the “regular” price, not a “on sale” price.
The objetive is: the products must show the “on sale” badge on the front end, just that.
I need to make the change in te products data and not make any change in the templates (this plugin nees to do the trik in any theme)