while I explored further onto this, here is what I found :-
- you need to select a template when you are to start creating a threat model.
- Stensils available will be based on the template chosen. so one needs to select a template that meets the requirement.
- while we are working on the model with a template, there is NOT a way to add any more stensils to the templete!
- you can add stensils or merge it with other templates to enrich it, when you just 'Open the template'.
- once you have added required stensils to your template, you can make this applicable to your model via the option 'Apply Template' after opening the model.
I found this working. may be someone could suggest a better way, if available.
Thanks,