Use the transactions BD62, BD79 and BD55 to setup and assign conversion rules for your IDOC fields and set a constant '/' for the infnr field.
Test if that is enough or you might need to take extra care for the name field of info record texts. I am not in a system currently, so I can't check whether long text name is based on info record number or only a mix of info record keyfields (without info record number itself).
You should also have a look into my blog Stumbling blocks in purchasing info record migration by LSMW IDOC method
You will probably hit the one or other issue that I described too.