OK, the link to the tool I used is now dead, but a little googling turned up this
. It's a bit more involved, but it's pretty direct. The alternative if you don't want to muck about with that is to restart an ME2 game with that character, sit though all the rigmarole of getting to the character creation screen then eyballing it so the custom face is identical to your imported face and noting down the face code. Once you have that, the rest is self explanatory.