Hi Assen,
I just ran a transaction in a test system, and I can get an entry in the work-list without an MRN or Preliminary Document Type. You need to make sure that the Customs Status is set to "unpaid" ('2'). Maybe you will need to achieve that in the ERP BAdI /SAPSLL/IFEX_MM0C_R3?
Please try again, and if necessary have a developer check in debug what is going wrong.
Regards,
Dave