I understand what you want to do. What you need is to create composite glyphs.
You need to use the Complete Composites feature. Unfortunately, since neither I nor Erwin know Korean there are no definitions in the CompleteComposites.xml data file for Hangul. If you intend to create multiple Korean fonts, it would be worth adding the code yourself. It would also benefit future users who want to create Korean fonts with FontCreator, but it would be time-consuming.
Otherwise, you can create composites with copy/paste.
- Select and copy the glyphs that you wish to combine in the glyph overview.
- Open a the glyph editor window for the composite that you wish to create
- Paste the clipboard contents
- Adjust the composite glyph members as you wish.
The method is described more fully in Creating Composites in the Home Edition.
If you want to try adding the code to the CompositeData.xml file, let me know and I will show you the method for a few composites to get you started.