yes, you can actually
but not through update of the existing mirror. 
tat'll simply erase all others except those u choosed to update, i've had
experienced it......
my way of doing partially update is creating a new mirroring project and set
scan rules to just get the part u want to update and then replace that into
the existing mirror, but the problem is everything will be re-downloaded for
tat particular part, for ur information. (it is better than updating the whole
mirror in my case, so it depends)