Yes, the symbol combobox is currently for SPICE subcircuits only. You should not use it for sNp files.
As a workaround, you can choose “edit” and in the “model” field append the following line:
“polar” 0 “linear” 0 “open” 0 “none” 0 “capacitor” 0
Note: The first character in the line above is a space!