FileSelectPopup does allow the user to browse to a directory and then manually input the name of a nonexistent file. You just have to make sure you pass VAL_OK_BUTTON or VAL_SAVE_BUTTON for the button label parameter (as opposed to VAL_SELECT_BUTTON or VAL_LOAD_BUTTON). If you pass either of the latter, the dialog requires the user to specify an existing file.
This doesn't seem to be documented. I'll make sure that point gets added to the documentation.
Mert A.
National Instruments