Is there an adapter that allows Yakima roof-mounted bike racks to fit
Thule brand roof racks?  The difference between Yakima and Thule roof
rack systems is that Yakima uses round cross bars and Thule uses
square cross bars.

Yakima bike racks attach to the round Yakima roof cross bars with
plastic pieces that clamp onto the round bars.  It seems that there
should be plastic clamps that fit square bars so the Yakima bike rack
can be used on a Thule roof rack.

I have not been able to locate such an adapter.  I have Yakima bike
racks that I want to fit on Thule cross bars.