I looked through Github, and it looks like you cannot currently import bitmap fonts from the editor. This Reddit post shows one way to get around the issue though, so maybe it's worth a look?
But from what I've seen (and searched), a .TTF file is for DynamicFont objects, not BitmapFont objects. If you have a .TFF file, I would highly suggest using a DynamicFont, since then you can load it from the editor.