The “problem” is that the default skysphere builds up the sky using a quite advanced material and lots of calculations in the blueprint.
So if you just want to swap the texture to a prerendered panoramic texture you should create a custom material (you could copy the M_Sky_Panning_Clouds2 material and remove all the nodes in it) and add your texture inside the material.
The go into the blueprint, select the sphere mesh and assign your custom material.
You will also have to go inside the Construction Script and break the link between the entry node and the “Create Dynamic Material Instance”-node to prevent the blueprint from trying to set parameters in a material that isn’t assigned 
Please let me know if that works for you, or if you want more info.