Without knowing the focal length, the exact lens added to the front of the existing one can't be calculated.
However, what you want is called a "closeup lens". These are usually single-element convex lenses that mechanically mount as filters. First you have to find the diameter of the filter mount on the lens. This is usually written on the front of the lens somewhere. You should also be able to look it up from exact lens model.
Once you know the filter thread diameter, you can get a set of closeup lenses that will fit your lens. Closeup lenses are usually specified in diopters instead of focal length. Start with a "+2" closeup lens and see how that does. You can readily find closeup lenses in the +1 to +3 range. +3 is about as far as you want to go for a single-element closeup lens because of abberations.