It's impossible to 'overwrite' a shown dimension in ProE.
However, you can 'overwrite' a created dimension in your drawing.
Simply create a dimension --> Change it's 'Dimension Text'. You'll see a @D in that window. Change that to @O, and typ then your text/parameter. If it's a parameter use a '&' sign before the parametername.
The result will be a dimension with a text or parameter that you prefer.
You could also change the name of the shown dimension to however you want the text to show on your drawing. In Wildfire, this is done by accessing the dimension properties after right-clicking the dimension. Then, instead of @O, as FDdesign suggested, use @S. This will show the new name you defined, not the dimension value.
I can only speak for Pro/E 2001. You cannot add a parameters to a dimension if you use @O. Pro/E places the exact text you type next to the @O even if it is a parameter. You would have to type the text seperately.