How to export Flat Pattern where outer polyline is joined to a single polyline? 

Not consisted of individual lines.
 

I made a Drawing which contains only Flat pattern:

Then I export the drawing to a DXF file. I do it like this, because this is the only way to have bend notes (angles) in Flat pattern DXF.

The issue is: outer polyline consists of a list of an unjoined lines:

Is it possible to join all those lines into a single joined polyline object? Like this:

Our local Tin Sheet supplier wants the outer polyline to be joined. Their CNC software requires this.
I can manually open each DXF file in Autocad, select all its lines, and hit "Join" command. But I am trying to avoid this manual work, because there are plenty of DXF files.

I googled a bit, and found two suggestions how to fix it:
1) System Options -> Export -> Enable Merging. I played a little bit with numeric value - from 0 to 100:

2) Right click to 'Flat-Pattern' -> Edit feature -> check Merge faces:

Neither of these two suggestions worked - the resulting DXF file still has outer outlines consisting of individual line segments.
 

I would be very grateful for any kind of help.