Automatic replace missing glyph from other Master

Hello everyone,

I’m building a font in several weights, and in Italic for each weight. The font is in Latin glyphs and Japanese.
I do not have italic in Japanese, then how can I say:

If Regular Italic Glyph is missing →
sub by the Regular glyphs

In other word, during the export, regular glyphs would automatically replace missing Italic glyphs.

Thanks a lot!

Edit: in other words, how to fallback to regular if italic glyph is missing? Is it even possible? Or should I just copy and paste the regular glyphs into the italic master?

We have a good solution in an upcoming version. In Glyphs 2, you will have to copy the layers over, best with a script.