Well, first you'd have to complete the conversion, then install the Attachments MOD. Then, you'd have to copy all the files from the smf attachment folder into the newly created phpBB folder. Finally, and this is the most tedious step of all, go through each post that contains an attachment and basically reattach it (just through sql, not actually reuploading the file).
PLEASE NOTE: I have left the coding community and no longer provide support for any coding I may have contributed to in the past. I apologize for the inconvenience and fully welcome anyone perfecting my code for the betterment of this community.