The temporary B-Rep capabilities are fairly limited and do not support the ability to offset a body. A workaround is to create a body in a base feature and then use features to create the offset. I tried some things interactively and found the Offset Face command does this when you select all of the faces of the body. Unfortunately, I also found that the API doesn't currently support the creation of the Offset Face feature (OffsetFacesFeature). However, I also found Offset command, which creates a new body and the API does support the creation of this feature (OffsetFeature).
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com