Answer:  It was complicated to do it, so we didn't!  We might be able to add this in the future.  There is a work-around: for the time being, you can manually convert fractions like "2 5/8" into ones of the form X/Y, which ConvertIt will then recognize.  For example, to convert "2 5/8" to a fraction of the form X/Y, multiply 2 times 8, then add 5 to get 21, then place that over 8, as in 21/8.

Explanation:  We wanted to allow measurements whose names began with numerals, and it was hard to distinguish those fractions from the names of those measurements.