Unfortunately, this is currently not possible due to game limitations. The mod works by using invisible inserters to put ammo/fuel into entities. Doing this with inserters rather than through scripting is what keeps the mod's UPS cost relatively modest in the late game. As such, the mod is bound by the rules of normal inserters, which use each entity's prototype data (defined on startup) to determine how much of an item should be inserted into any given inventory. For most ammo types and fuels, this happens to be 10.
I could probably add an option to use scripting rather than invisible inserters, which would allow this feature to exist, but be warned that the UPS cost of the mod will be much higher while this option is active.
I'll let you know if I'm able to implement an option like that. Let me know if you have any other questions!
