How to add red/green wire on belt without stopping it?
You can use the copy and paste feature. First, make an extra unused belt somewhere else. Place red/green wire on it and configure it as you wish.
After that, press Shift and Right click the belt. All settings were copied.
You can now Shift+Left click any belt and the settings will be pasted on it - even if it doesn't have any wires attached. Once you attach red/green wire, the pasted settings will take effect immediately.
The only downside is that it's impossible to see what the settings are (and whether you pasted them successfully) before attaching the belt.