Yeah! You can pursue Diploma courses in Polytechnics (Government/Private) after your ITI course. In some states like UP, you can get admission directly to the 2nd year of Diploma by giving JEECUP exam. Now based on your state you have to find out the polytechnic entrances and accordingly you will get the rules.

As you didn't provided your state so I cant tell you the name of the polytechnic entrance corresponding to your state.

To know more about Polytechnic you may visit the link provided below:

https://engineering.careers360.com/articles/polytechnic

I hope this answer helps. All the very best for your future endeavors!