Sketch can't do all the work for you
There's no way to make a compound shape into a single line drawing. That's true of every vector app I've used.
All is not lost
You have a simple vector object. Just cut it up and reassemble the lines to create the line graphic you're after. In this case, you just need the surrounding frame and the middle line. Those are easy to isolate once you break the shape apart.
Here's a quick illustration of the process using the mailbox icon from FontAwesome.
After I converted the font to outlines, I had essentially the same starting point: a solid shape.
- Split it's paths: Layer > Paths > Split
- Dump the fills and add borders.
- Trim down the internal shape (the front panel of the box) to an open line using the Scissors tool.
- Move everything into place and adjust the internal line's outer vectors to overlap the surrounding shape.