Well, you sort of can already. You can enter an equation instead of a hard number. If you have your units set to inches, you can simply type in "1/25.4" to get 1mm. I do that a lot to get a nominal dimension form limit dimensions.
Yes, but.....I guess I've never had it be much of an issue. You can also create parameters with conversion factors, and write relations. I did that for a temperature label to convert F to C, and then used the parameters values as driven text in a datum curve to show the value of the parameters. I made a family table of the temp labels, and it worked out pretty well.
Google Sketchup does this, and it's quite useful for modelling my house where I tend to measure existing things in mm, but I often think in imperial for larger (and less precise) things, or where objects like sheds are sold in feet and inches!
However, I've never really found in necessary in my job - as Frank says, the "xx*25.4" does the job on the few occasions it's applicable.
