Problem with adding schwa

How to add schwa without changing kernings? Please, help me urgently.

What do you mean? You have a font and like to add only one schwa glyph?

If you import an existing font, there are always some bits and pieces lost. Glyphs is not really intended for this kind of operation. This does not mean that you can try. Open the font, add the glyph and export it. Test the font, fix the things that are not right and export again…