Do you mean, can I have an expression read the value of a cell in an external spreadsheet?
If so, the answer is YES.
When in the expression dialog, while creating an expression and it comes to assigning the value or 'Formula' for the expression, select the [i][b]f(x)[/b][/i] icon at the bottom of the expression dialog and then select the 'Category' titled 'Spreadsheet' and you'll see a series of functions which will allow you look for and read values for individual cells of an external spreadsheet as well as create list expressions from a range of cells.
John R. Baker, P.E. (ret) EX-Product 'Evangelist' Irvine, CA