In Reinforcement Learning can I randomly assign next_states from the state space to my agent while creating transition set?
It depends on your environment. For example, If you environment is a straight line. Let's say that the states are 1, 2, 3 ... 10. The agent here can either move left or right. Now, if the agent is at state 4, and it moves right, according to the logic of the environment, next_state has to be 5. 

Now, if you include some sort of a condition that, if the agent reaches state 5, the agent will be randomly moved to any other position, then you can return the next_state to be any random number.

Long story short, the next_state is dependent on what the environment is.