This is only possible if the garment has a generous (extra fabric inside the seam).
When adding panels, ensure the fabric grain of the new piece runs in the same direction as the original garment to prevent weird twisting. Altered Clothing: Hip Fixes and Transformations...
Turn the garment inside out. Pin along the side seams, starting from the waist or mid-thigh and tapering into the existing seam. This is only possible if the garment has
Look inside the side seams. If there is at least 1/2 inch of extra fabric, you can gain nearly an inch of total room. Unpick: Carefully remove the original side seam stitching. Pin along the side seams, starting from the
Cut or unpick the entire side seam of the pants.
After any alteration, press your seams flat. This makes the difference between a "homemade" look and a professional finish.