It is not totally easy, but easier than making a new nut. You can loosen the string and move it out of the slot. Mask around it with tape. Put a heap of Baking Soda (not baking powder) in the nut slot. Add a few drops of super glue. Spray with activator it you have it (this will cause it to instantly set up). It not, then wait 30 minutes until the glue sets. Refile the slot to the desired depth.
This is an old banjo luthier trick. Arthur Hatfield, a very well know banjo builder, showed it to me when he was working on my banjo. The resulting fill is white and VERY hard. The baking soda crystals (Sodium Bicarbonate) are very hard, much harder than typical filler materials. It will last as long bone in a banjo, longer in a classical guitar.