Hello Aerial11,
I guess you have a valid URL for your document which you will attach to your Infopath form. Then you can accomplish this in two steps. In the first step you have to "stream" the file from your document library and in the second step you attach this "stream" to the attachment control in the form...
Even though I have not a complete example but I have a few lines of code which will help you to get started.
Extract a file from a document library:
SPSite site = new SPSite(http://YourServer/YourSite);
SPWeb web = site.OpenWeb();
SPFile file = web.GetFile(@http://YourServer/YourSite/YourDocLibrary/Document.doc);
System.IO.StreamReader str = new System.IO.StreamReader(file.OpenBinaryStream());
Attach (=Encode the file) to your file attachment control
http://support.microsoft.com/kb/2517906
With the encoder class in this article you can use the 'stream' from step 1 to add the file to your infopath form.
Hope you will find useful tips
Regards,