Dear Parikshit,
Before giving reply to your query, I need to give clarification on the difference between Offer Letter and Appointment Letter. Once the employer is satisfied with the candidature of the candidate, the letter of offer is given to the employee. In this letter basic details are mentioned like what would be the designation, break up of salary components, date of joining etc. Once the candidate joins company, utility of this letter cease to exist.
The job candidate joins the company, he/she becomes employee and that person needs to be issued a letter than mentions terms of employment. This letter is called as Appointment Letter.
It appears that you are talking about appointment letter and not offer letter. Yes, provisions of non-disclosure of information can be included in the appointment letter. Your second query is "employment contractual clauses". Please confirm, what information you would like to include under those clauses.
Thanks,
Dinesh Divekar