It doesn't change his wages or value on his player contract sheet, neither extra position nor special ability.

In a better market, there would be more demand in auction and thus higher value, but the market is imperfect enough that it isn't consistent. Without getting further into the economics, I would advise adding those extras only if they benefit you while you have him on your team. Don't worry about sellability, list for the max price and touch wood, it's ask luck from there. (Nobody beside Nordeus has the data to declare SA a better predictor of market demand than goals/match or form, etc)