Can I use direct contract for ongoing and long term work?

Sorry for confusing title, basically I have a direct contract with non upwork client

This is a very long term contract, involves doing batch of work and then getting paid for parts of work.

So, my question is, can I use fixed price contract, input amount of tasks I've done (in one batch) price for these tasks, and then create new milestone in this contract, and continue like this for a long time.

My main concern is that if I create a fixed price contract for current batch of work, I won't be able to create new milestones after contact was created