You can create some components which are not included in PDK. Some devices are not modeled or extra price if you want to use, but you can try to create layout and model them, but last is quite difficult I think.
For passive an example is the MOM cap, which doesn't support to use all available metal layers sometimes, or its breakdown voltage is not what you want and you can adjust horizontal distance between plates, but PDK usually doesn't let these as default option.
For active an example is the standard logic cell library, which is not given sometimes, or cell types are limited, contacts limited, etc. and you can set/create those.
And actually there are some tutorials available where people create layout first of circuits then schematic of the instance. LVS is smart enough to recognize active and passive devices. So, I can imagine maybe it is possible to create a donut shaped MOS for example if you want, until its layers can be recognized in the correct sequence.
Other question is how can it be manufacturable, unfortunately I have no experience with it. Maybe donut shaped CMOS is a default device in PDK, I have no idea about, it has just got into my mind.