Attribute product model numbers

Maybe I am missing something, but I can't figure out how to do this.

Example:
I am selling shirts - (a,b,c)
They come in four sizes (s,m,l, xl)
Each shirt type has it's own product number. a1 - a2 - a3 - a4 / b1 - b2 -etc...
a1 is small shirt a / b3 is large shirt b, etc...

Is there a way so that I have one item (shirt) with four attributes (sizes) that when selected adds the appropriate product number to the cart ( say a4 for shirt a-xl)?

This seems like basic functionality, am I missing it or is it not possible ?

Thanks

Sorry but, that sort of attribute functionality is not yet available.

Thanks Kim.

Im looking to do the same thing, but with trading cards. An attribute that is Mint/NearMint/Poor for one item. but 3 different prices. & 3 different QTY's