Copy/Paste to Create Composites
If you have the Professional Edition, Complete Composites can be used to create many composites, for example to create subscripts from superscripts. However, if the glyphs are not mapped, for example, if you want to create denominators in the Private Use Area from superscripts, or if you're using the Home Edition, then Complete Composites won't be an option.
However, composite glyphs can be created by selecting the glyph members in the glyph overview, copying them to the clipboard, then pasting them into the empty glyph where you wish to create the composite glyph.
To speed up the process of creating all ten denominators from the ten superscripts, copy them all at once, paste them all into the target glyph, then delete those you don't need.
Step One
Select all ten superscript glyphs in the glyph overview using Shift + Click, and Ctrl + Click. They will probably be in the order: 2,3,1,0,4,6,7,8,9 if the font is sorted in the standard order.
Step Two
Insert ten empty glyphs at the end of the font. Denominators intended for use with OpenType Features don't need to be mapped. Or, you can map them to code-points in the Private Use Area.
Open the first empty glyph and paste the ten superscripts from the clipboard. You will see a confused mess of glyphs like this:

- Pasted Composite Glyph Members.png (10.66 KiB) Viewed 32558 times
Step Three
To select the superscript zero cycle through the composite glyph members using the "w" shortcut key — the zero, which is the fourth on the clipboard will be selected with three presses of the shortcut key. Holding shift, press the down cursor about five times to separate the selected glyph from the group. If you get the wrong one at first, try again until the target glyph is separated from the group and in the right position on the baseline like this:

- Target Glyph Member.png (13.53 KiB) Viewed 32558 times
Step Four
Use the shortcut key "Ctrl T" to invert the selection to get this:

- Invert Selection.png (13.76 KiB) Viewed 32558 times
Press the delete key to delete the selected glyphs, and use the shortcut Alt Right cursor to move to the next glyph. Paste the clipboard contents again (Ctrl V), and repeat the process for the next denominator.
Using composites instead of simple glyphs reduces the size of the TrueType font file and makes the font easier to edit. If you later decide that the superscripts need to be bolder, for example, you only need to edit the superscripts, for the subscripts and denominators to be updated too.
Update: If a large numbers of glyphs are pasted, disable Automatic validation, and hide the Glyph Fills (shortcut Ctrl H in FontCreator 9.0.0.1916). Fill Outlines is a global setting that affects all Glyph Edit Windows.

- Hide Fills.png (13.24 KiB) Viewed 27698 times