Adding a leveling kit to a lift kit?
 

It has no blocks for a 6" lift?

Your biggest issue is that the Rough Country lift uses a knuckle for a 4"/6" lift (most do it this way. So basically a 6" lift is kind of like a leveling kit added to a 4" kit (not quite as extreme as most knuckles are around 5" height adjustment). So if you add more on top of this, you are getting into the same issues as adding a level.

I did look up the parts for rough country and the knuckle is shared amongst their lifts. Give them a call and verify their knuckle is 5" adjustment. If so you could add 1" to 1.5" and be at about the same as a 2-2.5" leveling kit. If it is a 4" adjustment, than they are already using the 6" lift like a leveling kit on top of a 4" lift.

Spoke to a RC rep. He said the knuckle is 5”. So i have the 6” lifted strut and wanna add a 1”-1.5” leveling kit to that. Do you think it would be possible and still use the factory UCAS?

Your question puts you in nearly same exact scenario to those wanted who add a 2-2.5" level to their stock trucks.

Note: I would not personally do this but it is your truck and stage 3 does sell a kit similar to your plan
https://www.stage3motorsports.com/ZO...-Lift-Kit.html