It depends on the release of WebLogic and the field table definitions used by WTC (and Tuxedo).
I believe embedded/nested FML32 fields were added to XmlFmlCnv in WLS 9.0 (but the doc needs to be updated).
Have you tried this yet?
The latest XmlFmlCnv usage documentation has not been updated.
If you provide an email I can send you some code used to test the
XmlFmlCnv class handling embedded/nested FML32 fields in WLS 9.0.
(i.e. without OSB Tuxedo Transport).