Retrofitting a roof rack
I took @Christopher Hunter's comment above an an answer, thank you:  I looked up Thule adapter and found a retail shop near my house.  I visited that shop today and was told that adapting was not possible:  the rack attaches to the roof bar in two places ... I could purchase a back "tie-down" that was wide enough to span the roof bar, but not the front.

I ended up buying two rockymounts brand tierods.  I have not yet installed them and can return them in case someone believes I chose poorly.