Unfortunately you did not say with which Idoc you have this trouble.
In general is LSMW able to add IDOC segments using the global function transfer_this_record
I described how you can create new Idocs from a file in my blog: LSMW migration with IDOC method and using IDOC as source Part2: Import
Someone else explained how to create a file from an existing Idoc inSave Idoc to file
I had no chance to test this as I am not with a system today.
Maybe it is even possible to directly use LSMW on WE19 to record what you did.