I think what you really want is shilling fractions rather than stacked
fractions. For information on both, see
http://www.mvps.org/word/FAQs/Formatting/CreateFraction.htm
FWIW, I discovered one day (quite by accident!) that if you select
Insert->Symbol, then Font: Palatino Linotype and Subset: Superscripts
and Subscripts there are symbols for the / and 1/ as well as
Subscripts for 0 - 9 and Superscripts for 0 and 4 - 9. No idea what
happened to Superscripts for 2 and 3, though! Unfortunately not all
of them "translate" (for want of a better word!) well to other more
standard fonts, but they do offer a passable simulation of TNR or
Arial (if you don't look too closely).
Another (faster but not "shillingly correct"!) work-around I've used
is to create a Character style which applies Subscript to all the
selected characters and raises them 1 or 2pt. (depending on size of
font I'm using).
hro