No. In previous versions you could overlayer SalesTable directly, creating your own fields in it. And why not the SalesTable.Extension too (but that would not make much sense).
However many of the extensibility possibilities were there already on AX7 initial version 2 years ago.
I suggest you familiarize yourself with the content of this page in order to get more shed on the issue: 
Also I highly recommend the development lessons in the Dynamics Learning Portal.