If you can calculate the scale from the trees file that is output by growtree, you may be able to edit the growtree figure file (which is produced by 'growtree -figure') and manually add the scale. You can get information about editing the figure file from the GCG user guide (or type 'genhelp figure'). However, this will be fairly complicated unless you are very familiar with the figure file formats.
There are other software packages that will do this including PAUP for the Mac and PHYLIP for Mac and Windows. Phylip may be available on the helix systems in the future; send mail to the helix staff via the ptr help system (type 'ptr' at the helix prompt) if you are interested.
A list of programs for Phylogenetic analysis is available from U. Arizona.