Hi @HarikrishnanBK,
Until the moment I don't think it is possible to customize RTE options provided in the email for Adobe Campaign Classic, I know that you can use the forms for UI features but not for the RTE options.
Please @_Manoj_Kumar_ correct me if I am wrong here.
If you need more detail you can share with us what you want to do and we can try to help you with it.
Best,
Celia