At this point in your code - Do base64.Rewind() - base64 isn't a stream object because of how you've populated it earlier in the DTL.

I'm guessing

target.{PIDgrpgrp(1).ORCgrp(1).OBXgrp(1).OBX:5(1).5} returns a string so you need to write that into the Stream:

base64.Write({PIDgrpgrp(1).ORCgrp(1).OBXgrp(1).OBX:5(1).5})

before you call your custom function (assuming you've already instantiated your stream).

Sorry, formatting messed up and I can't seem to edit...

If your message class is the one you want to convert just extend both XML and JSON ?

Extends (%Persistent, %XML.Adaptor, %JSON.Adaptor)

And (assuming your schemas line up):

Set tSC = tMsg.%JSONExportToString(.tJSONMsg)