Different ways to resize marquee selection

I’m finding this program very good for working with screenshots and mocking up different UI possibilities, but the marquee is sometimes hard to get right. If I want to expand the marquee by 1 pixel on one side with the “Add To Selection” mode, it’s not really possible because the “Move Marquee” function activates in the area I would need to click on. The Move Marquee function does actually work most of the time. You guys have done a great job on this program overall! 5 stars.

I would like to be able to put the program in a mode where I can resize the marquee itself with the selection/transformation handles.

Or it would be very nice to be able to use the Ctrl-Shift-Arrow Keys (or whatever modifiers are available) to expand one side of the marquee one pixel at a time. Left arrow would extend left side by 1 pixel, etc.

A little thing, but it might be easy to do.
Thanks!