SF has visited this territory previously. However, from personal experience I can remark that even RTW sleeves lacking sufficient underlap to make a true vent can be made to enjoy working buttonholes.
Removing the faux buttonhole thread and any traces or shadows of their existence begins the job. Next, remove the stitching that secures the sleeve lining. Open the sleeve seam. If sufficient underlap is available, the anchor points for the sleeve buttons need nothing save for attaching the buttons in a later step. If insufficient or no underlap exists, a skilled tailor can affix/graft a similar weight, color, finish, (etc.) piece of cloth where the underlap would normally be.
Then, the overlap part of the sleeve can be marked for working buttonholes to be cut - presuming the sleeve length is already correct. Once the working buttonholes are cut and finished, the corresponding anchor points for the buttons can be marked and the buttons sewn in place.
Whether or not the sleeve has sufficient underlap cloth or not, the sleeve lining is unlikely to have sufficient extra material in its circumference to reach into the overlap as it does on bespoke and MTM jacket sleeves ordered with working sleeve cuff buttons. So, the lining must invariably be sewn back in place with no real possibility of a break for the sleeve vent.
Since the sleeve is vented, but there is insufficient lining material to properly finish the vent's overlap underside (wow!), the overlap of the vent is usually invisibly tacked down to disguise the relatively narrow underside area lacking lining material in addition to obscuring the grafted material. So, you now have working sleeve buttons where (seemingly) none were otherwise possible. Proper selection of the material to be grafted, skilled tailoring, and the otherwise untrained eyes of the average observer will allow the finished sleeve to pass all but the very closest scrutiny.
My tailor has done this on a few RTW Oxxford suits and odd jackets that I just couldn't resist on sale. It's a great compromise if you're willing to compromise.