I must be confused by what you are looking for since this seems to be straightforward and only took about 15 minutes this morning. I used the long rifles upgrade as a base, changed the effect from “Attack Range Bonus†to “Attack Damage Bonus,†changed the name and such to Ducati for clarity, made the Town Hall have the ability to research it and let footmen (melee?), riflemen (ranged?) and priests (casters?) use it as an upgrade. It appears to work.
Look at the attached and let me know if this is what you are looking for.
Eric