You can do this, but the composites feature is best for creating accented characters áéíóú etc.
Pros: If you use composites, then need to modify the rectangle later, you only need to modify one rectangle.
Cons: In most fonts the characters are different widths, but your rectangle can only be one width. Also, you must compose the composites manually as the complete composites feature was never designed to do this. It is not difficult, just laborious.
Another way would be to just copy and paste a rectangle into each glyph from A-z and adjust the width to suit each letter. If you still want to use composites, take a look at my post in the Tutorials forum for making decorative fonts: New Fonts From Old For what you want to do, the rectangles need to be clockwise contours, while the contours of the letters all need to be reversed. Use the glyph validation toolbar to correct contour directions.