EDIT: Works with the Horse, but for some reason, I can't get the Bow to work when renamed.
You would need to extract any add-on you want to change and rename its folder (if it has a namecheck.txt, you have to modify this). Then, disable the original add-on.
In the new add-on, keep the datablocks as they are, except you should change all filepaths to the original files (example: "./arrowHit.wav" to "Add-Ons/Weapon_Bow/arrowHit.wav"), to avoid unneccesary re-downloading.
If you have any add-ons that require datablocks from default add-ons (Akimbo Guns, Tank and many custom weapons and vehicles) this would cause some issues (datablock overwriting ?), so you should do something about that. Open the server.cs in those add-ons, and make them check for the new modified add-ons instead. If you are modifying the Gun or anything the Tank relies on (Jeep, Rocket Launcher, Gravity Rocket), you also have to change the server.cs in the Guns Akimbo and Tank add-ons (if you are using them). Obviously, since you are modifying these add-ons, they too will have to be extracted and renamed following the steps above.