Say you held:

S x H Axx D AJxxxx C Axx

Now you want to play 6D if partner has support for diamonds and 3N if not.  

Partner has support. So we have (at least) an 8 card fit in Diamonds.

If, as you said " NOW YOU WANT TO PLAY 6 D IF P HAS SUPPORT-- 3NT IF NOT", the simple answer (and the logical ONLY answer) is just go to 6 diamonds, and enjoy it. (if you play transfers to minors, first transfer to diamonds, then go to 6 D.