Adding a control to CustomTool

Hi, the CustomTool has 8 numbers in the Controls tab, but I would need 9 numbers to manipulate a 3x3 matrix, is it possible to add an extra number to the tab with the Edit controls -feature?

Or alternatively, how would I get a value out from a cell in ColorMatrix, if I link an expression to a cell, all I get is "ColorMatrix1.Matrix"?