You can fill expansion joints with epoxy provided it is 100% solid and flexible. Do not use standard epoxy coatings as epoxy is not flexible. A 100% solids, epoxy is self leveling. As such it flows into the expansion joints, every nook & corner, and fills it up completely. The key word here is “flexible”. Anything used to fill an expansion joint that is not flexible, defeats the very purpose of an expansion joint.  A non flexible filling will simply crack when the expansion joint expands or contracts with changes in temperature. Elastomeric polyurea is also a good choice for filling expansion joints in a garage floor.