Hi Christian
As Ambrish mentioned, you don't need to conver the file into an XML as this is standard format and just needs to be placeed in ECC System.
The approach which we used in our case was to have two inbound interfaces, one is just to place the file with file adapter(without any mapping) and second one to pass the file name to the inbound proxy. Which indeed calls the standard program by passing the file name to upload the file in the ECC system.
Let me know in case you need more details.
Regards
Vj