Unfortunately there is only one type of downscaling available on the Workgroup Videohub. The downconversion feature on the Videohub is more for monitoring purposes than anything else.
Should you be using one of the Decklink cards that allows downconversion on output, such as the Decklink HD Extreme 2, you can set it to downconvert in either letterbox or anamorphic for SD. This can then be sent into the Videohub.
Technical Support Consultant
Blackmagic Design EMEA