Adding and joining a parallel line

Hello Wonderful People,

I have to disclose that I'm a total dummy when it comes to CAD. However, I need LibreCAD to perform a task that I think should be a somewhat easy one. I have a separate program that generates flat patterns as dxf files for constructing cone. The problem is the program does not account for a seam allowance which will usually be a 1-1.5" extension from one end of the generated flat pattern. I think I already figured out how to add a parallel line but I need to know how to extend the arcs of the flat pattern to that parallel line and make it all one seamless/solid/closed shape. The final result will be sent to a cutting plotter to be cut out of the materials I use in making the cone.

I have attached a sample dxf file as well as an image file for illustration. The dxf file already includes a line that I added using Draw > Line > Parallel. And the image illustrates what I am trying to accomplish by showing the required extension in blue lines (original pattern in black lines). The blue line will be the new border for the cut. I'd like to be able to make the increment in inches. I hope the request is not too confusing. Thank you so much in advance for your time! And Happy Holidays! Regards, octoeff

#12x15x8.dxf

Attached your file with the "arcs" extendeds
12x15x8b.zip

what you look is "Modify->Trim" (really is trim/extend)
the problem is that it does not work with polylines.
Yes, you do not have arcs in your drawing, both are pieces
of line (joined as polyline) creating an arc approximation