Adding your own parts and templates

Yes you can!

Parts
Adding parts is quite easy and best demonstrated with gifs:

Mounts(and connectors):
Mounts and connectors

Child objects are supported. The biggest/main object however, should be the active object. Take note of the origin of the main object, it should be placed at its intended pivot point but only if this point aligns with the cylinder. For mounts with an off-set, an arbitrary point on the Z axis can be chosen, see the 'arm 1' example (in the 'old' sub directory, example files) on how to approach this situation. Origins do not matter much for connectors.

Rods:
Rods

If the rod comes with multiple child objects, you only select 1 child object and lastly the rod. For rods the scene should be empty and only contain to-be rod objects to make this work properly!

Also: A rod needs its origin set to a number < 0 on the global Y axis. Any vertices 'beyond' the origin will stick out of the piston. These rods are not suited for the custom mode typically. Rotation should be applied after rotating the cylinder that will be your rod.

If you make a part with these features, the proper collection structure and/or any name conventions are added for you.

Templates
At the bottom of the documentation page on blendermarket there is a reference sheet describing what's required/possible. I tried to keep it as simple as possible! Most of what you see in a template is completely optional. I use drivers that derive input from an empty its location for some templates.