You can assign a variable to a drive dimension (see "Assign/deassign a variable to/from a drive dimension") using the same procedure you just learned for modifying a drive dimension, or by using the Assign button in the Spreadsheet dialog box.
Regardless of the method you use, the dimension immediately assumes the value of the corresponding variable. The name of the variable, rather than the actual dimension, is normally displayed on the screen (depending on the settings under Show variables as in Text category of Document Properties).