To join dissimilar duct which are in line

Select (Connect Ducts/Pipes), and key in a tolerance exceeding the center line offsets.

Note: Note the tool setting implies an elbow connection. Since the Connect tool uses automatic placement functionality, the application knows there is a change in size and shape, but not in direction.
Select the two ducts.
The ducts join with a rectangular to round transition.

Note: Because the tolerance value exceeds the center line offset, the ducts satisfy connectivity.