Add a property to a item or type

Do you mean something like this:
items managed by UI:

Switch OD_Switch_Lucht        "Vijver luchtpomp"				<light>
Switch OD_Switch_Water        "Vijver waterpomp"				<light>
Switch OD_Switch_Uv	      "Vijver UV licht"					<light>
your code goes here

And the definition of the actual switches are like this:

Switch	Klik21	"Vijver luchtpomp"			<light>	(gOD_Switches)					{ rfxcom=">18522370.5:LIGHTING2.AC:Command" }
Switch	Klik22	"Vijver waterpomp"			<light>	(gOD_Switches)					{ rfSwitch	Klik21	"Vijver luchtpomp"			<light>	(gOD_Switches)					{ rfxcom=">18522370.5:LIGHTING2.AC:Command" }
Switch	Klik23	"Vijver UV licht"			<light>	(gOD_Swithces)					{ rfxcom=">16979794.1:LIGHTING2.AC:Command" }

Then in one of my rules, I use this for setting the actual switches to the status specified by the UI:

    		Klik21.sendCommand(OD_Switch_Lucht.state)
    		Klik22.sendCommand(OD_Switch_Water.state)
    		Klik23.sendCommand(OD_Switch_Uv.state)

And this for turning them all off in one hit:

		gOD_Switches.sendCommand(OFF)