Using epoxy compounds to fill gaps and cracks in your hardwood floor is generally not a good idea. Epoxies have exceptional bonding characteristics, which is actually undesirable for hardwood floors, as edge gluing of the boards can occur. When multiple boards are glued together, this prevents free movement of the individual board. The edge glued boards now move as a large panel. Instead of having very fine cracks between each board, now the larger panel will exhibit a very large single gap. This single gap is the sum total of all the edge glued boards.
Filling gaps and cracks is best left to the flooring professional when your floors are getting refinished. The flooring professional will use the proper know-how combined with wood fillers which are designed for the wood flooring industry.