Yes, you can use a small amount of oil paint to add colour to plain encaustic medium.
When mixing oil paint with encaustic it is important to understand oil and wax relations see this Wax/Oil Ratio diagram from R&F.
There is a danger (the painting won’t be archival), in making a mixture where the amount of oil and the amount of wax are equal. Do not add more than 25% paint to 75% medium or you will end up with a wax that won’t harden. Some artists put oil paint on paper towel or cloth to absorb some of the oil before adding it to the molten medium.
Glazes of colour can be made using a very small amount of paint to the encaustic medium.
