In you dimension settings, go to modify (whatever style your using) and pick the tab primary units. Find the measurment scale box and enter .03937
Then in your units on the drawing itself, use demical to whatever decimal place your heart contents.
IFRs is correct, Autocad doesnt draw in mm or in, one to one is one to one. When dimensions objects by telling it divide this value by .03937 and display that number, which will show the metric equiv. If your having problems with dim lines being too big and such, you will have to go in on the dim modify and change all the values such as leader size, offset to metric values.
If it was me, I would do this anyway, and when I would come accross it in another drawing, open that drawing up with your metric settings, CRTL+C the dim, go to the drawing reworked and just hit CTRL+V. You can cancel immediatly and the dim never gets placed in the drawing but the style does, and its a matter of match props then.
Hope this helps.