We have a 2 year old home with insulated flexible ducts from the plenum to the registers. Is it possible to install manual dampers in these ducts? The number 6" is written on the outside of the duct insulation. I assume that's the size of the flexible duct.

Is the flexible duct cut, then a damper inserted and joined with the ends of the cut flexible duct?

I'm trying to better balance our system for winter usage.