Hi Olivier,
I looked into the code, and unfortunately the answer to your original question is "no".
The extractor (Function Module /SAPSLL/BW_CD_3_CUHD_EXTRACT) only selects documents with /SAPSLL/CUHD-CDTYP value 'A' or 'H' (Customs Declarations or Transit). Document Type DDOCI necessarily has value 'B' (Protocol). I can't think of any simple way to overcome that issue.
Regards,
Dave