Since I am not sure, in Numbers terminology, what a worksheet is the question is unclear. In Numbers here are the relevent terms:
A FILE is stored on your hard drive which contains a DOCUMENT which contains SHEETs which contain TABLEs, GRAPHICs, TEXT. TABLEs contain CELLs which contain text, dates, numbers, formulas.
If you are asking whether a cell in a particular table can refer to another cell in a different document then this is not possible in Numbers. A cell can refer to a cell in another table of the same (or differerent) sheet so long as the sheets are in the same document.
I hope this help clarify. You can download the free Numbers Users' Guide which can be very handy too.
Page 126 is where the details regarding cell references starts.