Yes, demo version allows diode modeling.
Here is what you need to do:
Launch Model Editor and do File>New
Do Model>Copy From > Enter new model name and browse to library where you have 1N4148 model and select 1N4148 from list
Say OK
Now modify the model parameters as per your requirement
Save this library (say new_diode.lib)
Use File>Export to Capture part... to create symbol for this newly created model
Place this symbol in schematic
Bring up simulation setting dialog and add this library(new_diode.lib) under configuration setting TAB in library files
You should be able to simulate design based on this newly developed model