I mess around with sewing, hemming and repairing and have even made some stuff too. I think I know what you are asking, but to know the answer you have to ask yourself what diameter you need the pants to be at the point where you put the two sections back together. If the diameter of the lower part is fine then you can open the seam on the upper part and taper a dart into them so the couture down to the lower part. (Easier said than done, but the way, because a straight line can make the pants look "blossomed".) If you have been lifting weights for some time and the diameter of the lower part is too small for your leg at that point then you have another problem that would be difficult or impossible to solve unless you are a much better seamstress than me I.
Good luck. By the way, I have done a lot of sole searching and have found I enjoy playing around with such projects, but in the end it isn't really productive if my time is worth anything. You seem to have thought this out carefully, that's important because sometimes I later figure out that I made a simple job much more complicated than necessary. What are the pants used for? Mountaineering, Skiing, Rock Climbing?