To add an outline to your custom font:
- Import your desired font (see here for importing).
- In the font editor, select the page(s) of font you want to modify.
- Click ‘Export’ and choose a destination for the exported file (the file will export as a .tga).
- Open and modify the .tga in your editor of choice (I use Gimp).
- After making all of your modifications, in the UE4 font editor click ‘Update’ and select your just modified .tga.