Isn't that, in effect, what I did when I added the glyph to the kerning class?
But this doesn't change the plain text in your kern feature.
If, say, you change a kerning pair in the Kerning dialog, you will want to rebuild the feature, i.e. re-generate the text in
feature kern{...}kern;.
If, on the other hand, you only add a glyph to an existing class, and you do not change the values of the affected kerning pairs, and you have
Preferences > General Options > Unicode and Opentype > Add all glyph classes... turned on, then the class definitions will be updated automatically and you do not need to regenerate the feature text. But then again, it wouldn't hurt if you did it anyway.
So, if in doubt, let it rebuild the kern feature.
