Dear Sandy ,
I can give a workaround for this ...
I am playing career mode and I am an off spinner (Finger Spin).
I have created some custom fields for my county First class game. I used them when I bowl.
But to my surprise when I came to bat , the AI bowler who was bowling to me was also a finger spin bowler and AI was using the same fields which I have created.
So my point here is try to create some custom fields with Long Form , Medium Form, Short Form and to bowler type which you want Fast , Spin etc.
May be AI will choose the field one of them you created.
I have created few fields I don't think there is any field share option in DBC17, but I can upload it over here.