Hi Anton, if you're finding the past too thick to disperse into your resin mix, it might be that you're storing or using it too cold. Like epoxy itself, our pigment pastes are fairly sensitive to temperature when it comes to their viscosity. If they're cold, they will feel particularly thick, so the first thing would be to check the temperature you're storing them at.
If this doesn't solve the problem and you are still keen to 'thin' the pigment paste for future use then yes, in theory, you could add a small amount of additional epoxy resin (resin side only) to the paste in order to dilute it slightly. However, it's important to understand that the percentage of epoxy that's already been used to disperse the pigment is very small, which is why we don't suggest you need to allow for it when calculating your hardener ratio in your resin mix. If you start to add significantly more resin to the pigment paste then you'll be getting into a territory where you do need to allow for the extra resin in the pigment.