With your first baseman Stovey listed fourth as an OF defensive sub it is not likely it will happen too often, but regardless of where he is in the OF defensive hierarchy, all you need to do is put the bench player as listed right after Stovey.
Or even before him - I admit I am a little unclear about whether regular starting players need to be listed first or not.
So when Stovey goes to the OF the person you want should be the defensive sub at 1st base. You might want to click the function for "double switch" in the Manager's Center where you set the managerial style and put it on "Aggressive".