It would nice to see a way to combine your low level epic troops to make them stronger for your hero or a way to feed those low levels of epic troops to level one hero’s to boost them as a stronger weak player
Once your stronghold is level 10 you can convert a building (recommended forge) to barracks to combine troops
As @Talisax noted, you can feed troops to other troops to level them up. What you ask above, though, isn’t implemented: one uses heroes to level up heroes, troops to level up troops.
Note that you can turn recruits that you gather in the world into heroes in the Training Camps. The only ways to collect troops, though, are as troops on the map or from a summons. Troop fodder is thus scarce, so the last thing I would want is to use those to level up heroes.