Depends which model of Yashica rangefinder you are using. On the earlier models, the light meter is just below the ASA/ISO dial. On the later models, the light meter is built into the lens rim.
If you have the earlier version, with the Above The Lens (ATL) light meter, then you can simply place an ND filter over the lens. The way that would work is that your ND filter blocks light through the lens, but does not block light to the light meter. So if you used a 1 stop ND filter, and had your ISO dial set to 800, then you could process your film as if it were ISO 1600. In order to get to ISO 3200, you would use the ISO 800 setting, and a 2 stops ND filter. This method works with the Yashica GT and Yashica GS.
If you have the later GSN or GTN, then the meter is built into the rim of the lens. The idea was that if you used a filter, then the light meter would automatically compensate for the filter. On these cameras, you cannot trick the meter to use a higher ISO film.