everything you said made 0 sense to me, you might as well explain quantum physics. export the original texture? How? No idea what paint3d is, or how to use image editing tool. I have no idea how to import your textures. Like I have 0 idea what any of this means.
in the character maker choose pants you would like to use, like i used in (1)
at point (2) you can set the main color of the pants, which you can see at (3)
if export the original texture via (4), you will have the current color in that file.
so rather choose a color you'd eventually have for the model at (5) and make it fit to your liking, as you can already see at (6)
then you need to export the original file in (7), as it will have your chosen color saved.
if you have exported the file from both (4) and (7) you will see the difference.
we will want to work (7) of course.
now you need a image editing software that is capable of transparency. win10 already comes with it installed and it is called "paint3d"
if you don't have that, you can easily download and probably should, because i will continue this guide with paint3d.
i have a german version, but just follow the red marked areas and you should be fine.
first you should click on the icon in (8) so on the right side you see the panel in (9) and check transparent background on.
the texture itself is a flat picture which will be encasing the mesh, so you need to know where the pants actually are. in our example (10) is the front side of the pants and (11) the backside.
in the next picture you can see that i have cut off a square of each side, which will be resulting in the right leg later on.
next you import your altered texture file with (12). important here is that you check (13), as otherwise the default texture will also be loaded, which still has both legs complete.
if you have done everything correct, it should look something like (14)