Importing a custom .png file as a glow effect? No, there isn't such a feature. You can choose either one from the offline gallery or download and apply one from the online gallery, importing glow effect like it is possible with buttons isn't possible and I also support adding this to the app.
I' m the developer of an app called SoftKeyZ. You can find the application's xda-thread here: Click!
If you would like to share Navigation Buttons (which you created) with other users of SoftKeyZ, just reply and attach your pack to this thread. The "pack of buttons" will be reviewed and then added to the online gallery, that can be accessed by all users of SoftKeyZ around the globe! :good:
For a pack of buttons to be added it has to:
contain at least one button (home, back, ...) or touch effect (touch effects won't work on Android 5.0+)
provide a transparent preview picture of 144*[button count*144] pixels
have a name in a text file
have the name of the author in a text file
be packed in a zip-file (see example zip)
every button needs to be 144*144 pixels or bigger, but keep the ratio (1:1), padding should fit to the other buttons.
every button needs to have a transparent background!
as file name for the pngs better use "home" instead of "SomeDesignHome"