The sleeves on my Rab Boreas were so long I added some. They're great.
I thought about using the button-hole setting on my machine, but the longest option was too short (I made them 2.5" long (ithink)). I sewed a line of stitching around the long rectangle of the shape I wanted, then went over all four sides with the densest overcasting stitch my machine has, using the narrowest stitch length available. Cut out the middle, lightly melted the edges. Done.