Dear Vivian, First and foremost, you need to be clear about the meanings of a few definitions. My clarification is as below:
1. Addendum is used to add or delete certain clauses from a contract agreement. While the grammatical use of this word might be correct, this phrase is not used to inform the change in conditions of employment.
2. You cannot insert a policy in the contract agreement.
3. When an employer issues an appointment letter, it is a contract under the provisions of the Indian Contract Act, 1872. Nevertheless, contracts are used to do business with contractors, service providers, etc. When dealing with employees, the term used is conditions of employment.
4. For your convenience, issue a supplementary letter to each employee about conflict of interest. Take their signature and retain the delicate copy in the respective employee's file. However, it depends on the strength of employees. If the strength of employees is in thousands, then you may just obtain acknowledgment from each employee for having "read and understood" the contents of the policy.
Thanks,
Dinesh Divekar