Yes, making a shoe fit well is relatively simple. Anyone with a C. Ped (C) or C. Ped MC designation should be able to do this simple shoe modification. Adding adequate padding under the tongue fills up the space from the top which makes for a more accurate fit rather than adding an insole which makes that side a bit higher and can potentially cause back pain.
If the foot is significantly narrower then padding the sides of the shoe makes for a good fit. The other option is stretching, buy the shoes for the smaller foot and have the other shoe stretched to fit, width is much easier to stretch than length but it is possible to make a shoe longer.
Also, some materials will NOT stay stretched no matter how much stretching is applied to the shoe. Leather is one material that will stretch and stay stretched every time.