There seems to be little documentation on how to use the CONVERT funtion in Numbers (both for Mac OS and iOS). After finding nothing in Apple's documentation and after a search on the internet and within Apple's support pages I was about to give up when it finally occurred to me that I should look and see how Excel handles its CONVERT function. Well, it appears that Numbers works the same way as Excel in this regard and the key is knowing what shortforms or abbreviations are needed in the "from unit" and "to unit" arguments. For example, to convert from inches to millimeters you can enter the following (where the first argument can be a number or cell, etc. and where in all of the following examples I just use the literal number "1".):
CONVERT (1, in, mm)
That may appear to be obvious, but note that capitalization (or not) counts. Thus, the following will NOT work:
CONVERT (1, In, mm)
Or, how about pounds to grams? This is how you do that (maybe not so obvious):
CONVERT (1, lbm, g)
Unfortunately, not even the Numbers '09 User Guide lists any of these unit shortcuts. However, if you search the internet you should be able to find a nice table of unit shortcuts that work in Excel and many of these appear to also work in the Numbers app (for both Mac OS and iOS).