I have nrf9160 DK.
I found there is u-blox driver available. Is it possible that the 9160 modem can be added an interface as a modem using u-blox? If so, there is any application note or any other resources?
What is it that you are trying to do?
The nRF91 has an application processor, so for most use cases, there is no need for an external processor.
If the internal application processor is enough for your use case, we have several libraries and samples on how to use it in our nRF Connect SDK (NCS).
If you only want to use the nRF91 as a modem, we do not have full support for that yet. However, it is planned for a future release.