First unless Colorado requires pistol registration it is not registered. There is no federal registration and there is no such thing as a "pistol" lower. There are lowers sold as pistol lowers, even stamped PISTOL or PISTOL ONLY but that is just a manufacture/marketing scheme just like the "for military or law enforcement only" Colts.
Unless there is a specific state law to the contrary, any lower can be built into a rifle. A pistol can even be built into a rifle as long as it has a 16" or greater barrel.
The receiver extension is not considered a buttstock so technically as long as you don't have a buttstock that can be ready installed on it then the stock carbine type RE is legal to use on a pistol, it is very ill-advised, but technically legal.
If it has a buttstock on it and a barrel under 16" then it is a SBR.
So in answer to your question, simply put a 16" or greater barrel on the upper, then put a stock on the lower (in that order) and you have a 100% legal rifle.