I have an new 870 tactical express synthetic.Can i add the tactical choke tube without threading the barrel.The barrell does not seem to be threaded

The Model 870™ Express Tactical with 18 1/2" barrel comes with an extended ported Tactical Rem Choke.
Are you saying you did not get this item when you purchased your new gun. It should have been installed on the barrel already?

Are you certain it is the express tactical or is it the express blackhawk spec ops or express synthetic which only has a 18" fixed cylinder choke? If either one of these is the case you cannot add the screw in choke.
You either need a new barrel or see if a gunsmith can thread the one you have.

Thanks Have the Synthetic