You cannot -- or at least Mac does not permit selecting folders at all (only moving between them) for uigetfile()
There is uigetdir() but it does not permit multiselection.
It is possible to work on the Java level to invoke the key routines that uigetfile() and uigetdir() invoke, but turning on multiselection on the folder browser. I just tested it the possibility interactively and was able to select multiple folders.
