You cannot "add a radiobutton after click new item button". All the forms for data list items are pre-defined - the end-user cannot simply add a new field. You'd need to provide a custom data list model as an extension alongside the relevant form configuration to add a custom type of data list that then contains a radio button - for whatever you want to use it for.
In the future, providing as much information about your use case will help ensure you get good answers.
