As all valid upgrades are defined in an enum here https://github.com/Direwolf20-MC/MiningGadgets/blob/master/src/main/java/com/direwolf20/mininggadgets/common/items/upgrade/Upgrade.java, it's currently not possible for other mods to add new upgrades. As mentioned in a comment in that file, ideally Upgrade would be a class that other mods could instantiate.
Changing this alone would not allow for extensive addon support, it would only let higher tiers of Fortune and Efficiency upgrades work; a way to specify an enchantment to be applied to the tool used to simulate breaking a block tied to each upgrade ID would allow for much more possibilities.