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.
If you have enough time to go through many threads in this site which end up questioning the exit procedure on foreclosure of the contract of employment by the newly appointed employees, you will find many complaining about the discrepancy between the offer letter and the subsequent appointment order regarding such conditions. In fact the offer letter is the original document containing the actual conditions of the contract of employment that gives the prospective appointee sufficient time to consider the offer of appointment before actually joining. Therefore, if the condition of NDA you mentioned is genuine in relation to the nature of the position offered, certainly it should be mentioned in the offer letter itself.
There is no wrong in mentioning the NDA requirement as body content or annexure in the offer letter.
It is a standard practice in many industries to intimate the offered candidate (formally) the requirements to fulfil on the date of joining and during induction/probation.
Sharing a specimen copy for you to refer.