Hi, Francisco -
I'm pretty sure there is no automatic way to accomplish this.
Theoretically, you could do this via an editor (manually) or a program, but you would have to identify and remove ALL of the DW source keywords, options, values, etc. that have been added to the DW syntax since version 6, and also make sure you saved the downgraded source file using ANSI encoding, as DW source was changed to Unicode16LE encoding beginning in PB 10. It's probably much, much easier to create/replicate the desired DataWindow in PB 6, provided you have a working copy of that version's IDE.
Regards, John