I'm no expert on auto stereos but I"m sure you can't simply connect a Sirius head unit to the base radio. The actual Sirius module is a black box installed somewhere under the dash and connected to the head unit through various factory harnesses and computers.
Beginning with the '06 model Rams the electrical system uses the CANBUS electronics that transmits data signals back and forth over electrical wires. The entire electrical system of '06 and later trucks is interfaced and controlled by computers including the sound system.
The only way to have Sirius is with an aftermarket radio head unit or an aftermarket add-on converter unit that transmits via the fm antenna.
Personally, I would not buy an ST model truck. There are far too many conveniences and comfort features provided in SLT models that I am accustomed to and appreciate.