The best way to do this is to have your CATIA Administrator setup a drafting standard in CATIA that will default to all dimensions that way you want them.
In the meantime, you can change dimensions to make them dual:
1. create the dimension
2. right-click on the dimension and go to Properties, and go to the VALUE page
3. turn on the DUAL DIMENSION option
4. choose a dimension style for the primary value (NUM.DINC is often used for inches)
5. choose a dimension style for the secondary value (NUM.DMM is often used for millimeters)
6. set the decimal places and other properties, and OK when done