I don't know what you do to import the json into the datawindow, but you would have to do the same thing for the nested dw.
To get a reference to the nested dw, you can use GetChild().
You might have to temporarily set the processing to 5 for the GetChild to work if you're dealing with nested reports:
string ls_restore
datawindowChild ldwc
ls_restore = dw_1.object.datawindow.processing
dw_1.object.processing=5
dw_1.GetChild("name_of_column_with_dddw_or_report", ldwc)
// now you can treat ldwc as if it's a normal dw:
.....
ldwc.Import......
// restore dw_1 to its original type:
dw_1.Modify("Datawindow.Processing=" + ls_restore)
Something like that?
"I've been able to work around the getchild() limitation with the following technique:
DWObject ldwo_nested
ldwo_nested = dw_1.object.nested_report"
So that might be another workaround, though not fit for dynamic stuff, like what I had to do to apply translations.