Oh. Well, that's unfortunate. I thought that it was just a matter of typing a few hundred lines of code in Notepad, change the file extension, and move it to the correct folder.
There may be some way of force-changing it behind the scenes but certainly wouldn't be easy at all, as not only would you have to have to have it implement that map, but you would also have to have some way of making US2 apply that on each planet you would want it on instead of all or none, which that plus any technical problems you might have basically turns this into one heck of a hacking attempt, since no, there is no way to do it currently and there are no modding tools to make it happen yourself without legit force-changing stuff, which in itself would be very difficult because the source code isn't available and probably never will be.
Tl;dr if theres some way of doing it that I dont know of it would probably be considered hacking and would also be almost impossible.
Now, I have heard (probably from the roadmap) that much later on they hope to implement custom model importing, whch I assume will also let you import the textures for said models, in which case it would only be logical that they would add planet texture changing at the same time, so the devs themselves may make it happen, we'll just have to wait. And if you want to know how to actually create the map that you would want to use in the first place, thats a whole other subject that may be better to just pm me about because I do have some knowledge of that.