Since the 4Runner does have a DSG tranny, I would not be surprised if the steering wheel could be retrofitted with paddles. However, this is purely a guess on my part. Your best bet to determine the feasibility of this might be to contact an organization that adapts vehicles to accommodate people with specific handicaps. I don't know of any, but your Toyota dealer might.
Having said that, I would think that paddle shifters in a 4Runner might lead to more frustration than satisfaction. Paddle shifters are usually reserved for performance cars that want/need lightning quick shifts and response from associated transmissions. Some 4Runner owners have already commented on a lack of throttle response and/or tranny lag (to throttle response) when driving their 4Runners. This is why I think paddle shifters might lead to a level of frustration. That, and the curiosity if whether the 4Runner's tranny is stout enough for the response normally desired from paddle shifters. I'd stick with the DSG.