The armholes on the chupa are sort of curvy; if you want to add sleeves to that, it's going to be like doing the sleeves on a men's dress shirt, and easing those curvy pieces tends to be tricky for beginners. The sleeves and armholes on a kimono are all just straight lines, so that would be much, much easier for you to do -- if you've made a skirt, you can doubtlessly handle sewing straight lines! :) So if you have to do alterations, taking something like a kimono pattern and making the sleeves a little bit longer or narrower would be pretty simple -- you're just taking a big rectangle and changing its dimensions before stitching it on to another straight piece of fabric; constructing a curvy sleeve from scratch and making it fit onto the curvy armholes of the chupa would be trickier drafting as well as trickier sewing. (Also, the way the shoulder seams fall on the sleeves in the screenshot, it looks more like the boxy cut of rectangular sleeves, not the closer curvy fit of a set-in sleeve.)