Can we combine Agile and Waterfall together?

The short answer is yes if you're in a certain transition period. That period could be lengthy.

The goal of an Agile Transformation is to move out of the traditional waterfall process and embrace the Agile approach as quickly and successfully as possible. You will find the traditional system less desirable once you've transitioned to the Agile mindset and the Agile approach.

This is like buying a new Tesla car when you already have a 2005 Toyota car. It takes time to transition to and get used to the new car because the mechanics and drive are quite different. You can use both cars if you want to. However, once you're fully into your new vehicle, you will find that it provides significant advantages and little reason to switch back. Once you love your Tesla, it is uncommon to drive the 2005 Toyota again other than to relive some memorable moments. :)