If you’re editing an existing weapon that you have unlocked online. Yes, the mod can work online depending on what exactly you’re doing. Anything with editing weapons should be fine though you just need an extra command.
You can get started by looking into existing mods on Nexus and asking questions in the modding discord server (I believe a link is in the OP, but it’s also on https://notex.app). While you’re on that site you can also pick up a copy of RPKG Tool. It’s also good to familiarise yourself with Simple Mod Framework. There aren’t many tutorials on modding Hitman 3, you kind of have to probe everyone’s collective knowledge and ask well structured questions. I cannot stress that questions point enough.
There are also many tools, not just one… for now.