Dual-Unit Dimensions

I'm trying to get a single dimension stencil to display it's length in
both imperial and metric formats: 1'-3" [0.38m]

In this case, the feet-inch is still the more-valid measurement (it's
the "true" value, while the metric is serving as an approx backup), so
changing the base measurement units of the drawing is not really the
direction I want to go. Oddly-enough, it seems to be a relatively easy
excercise to go the other way (entimeters as the page unit, plus
displaying the feet-inch equivalent) as Visio 2002 Pro has a Feet-Inches
output format. Is it possible to add additional Formats to the
Insert|Field dialog?

I'm pretty sure that an inserted field in the dimension's text block is
the most promising lead, but I don't know enough Visio formula mechanics
to be able to successfully convert the inch value to centimeters.
(Visio seems to retain the units designation in simple multiplication
operations, and I haven't found any formulas that might convert units of
measure.)

As I can immagine that a solution should be pretty simple, if someone
can give me a small hint, I'd much appreciate it.

Thanks.