Yes Absolutely! You can do B.Tech after Diploma/Polytechnic.

you must appear CEEP (Common Entrance Exam for Entrance into Polytechnic) Similar to what happens in the Councelling after CEEP same thing going to happen after ECET as well. Based on your Merit you will be choosing your Engineering College.

In order to get into the National Institutes of Technology (NITs) and various other colleges, you'd have to write the JEE-Main exam conducted by CBSE every year.

A merit list would then be prepared based on the JEE-Main score. Based on this particular merit list and student's preferences, students are given admission into an appropriate college and stream.

All the very best and good luck.