First, which version of FontCreator are you using? Home, Professional, or Trial version?
- Dz and dz with caron are mapped to glyphs in the Latin Extended B character set as you noted. However, Dz and dz with macron have no mapping position assigned in Unicode 5.2 as far as I know. That means you should map them to any position you like in the Private Use Area, which starts at $E000. Some PUA code-points are used internally by FontCreator for low profile accents, so I suggest mapping your custom glyphs starting at $E050. It is not difficult to change the mapping later if necessary.
If you’re using the Trial or Professional version, add as many characters as you need from the Insert, Characters dialogue, e.g. from $E050-$E05F
Then see this Tutorial post on Creating Composites in the Home Edition to create your composite glyphs in those new characters. Ask in this thread if you have problems.
- See this Tutorial to learn how to combine contours. Ask again in this thread if you have any problems.
I attach a rough and ready example font to show how it could be done.