I think that it depends on which B2B Protocol you expect to use. Protocols like EDI or RossettaNet do have strict definitions on the payload.
But ebXml or AS2 do not specify the payload, but would expect it to be (any) XML. Since JSON is in fact plain text, as XML is, you could package it in a simple xml element and send if via ebXML. But if you take a look at: https://docs.oracle.com/cd/E17904_01/user.1111/e10229/b2b_intro.htm#XBBUG1144
Then you see that you could use Custom (non-XML) as a document protocol. If you combine that with HTTP as a transport protocol, I'd say it should transport JSON pretty well.
Could you elaborate on your usecase?