Although it can be done, thinning epoxy by adding a solvent is rarely recommended. Although it may act as a thinner, depending on the solvent and the epoxy formulation, even drastic changes in the characteristics of the epoxy can happen.
For example, adding lacquer thinner significantly reduces epoxy's compressive strength significantly compromising its viability as a structural adhesive. Adding solvents like Acetone can extend curing time which makes the work less predictable. If the solvent doesn't evaporate before the epoxy hardens it can cause the epoxy to shrink and crack over time. Solvents can also change the color of the cured epoxy.
That's why the recommendation for thinning epoxy is to either heat the resin and hardener before mixing and/or heating the substrate materials. Using heat doesn't compromise the characteristics like adding solvents does.
That being said, although I don't add any solvents, I do often add one small drop of acrylic paint to my epoxy to color it when I am using translucent plastic blanks. Besides the color, it does seem to make my epoxy take significantly longer to harden up.
Knowing the potential ramfications, you will only know for sure if you give it a try. Because of it's high evaporation rate I would suggest Acetone.
Dave