Dear Geetha K,
The conditions of employment mentioned in the appointment letter are binding to the employee. Therefore, you do not have to enter any special agreement beyond the appointment letter. In the paragraph on "Conditions of Separation," you may insert the clause on "Continuation of Employment till the Completion of the Project."
However, please check with a legal expert on the validity of inserting this clause. Generally, in the appointment letter, a clause on the notice period is always mentioned. The purpose of the notice period is to give an employer breathing space to search for a replacement for the resigning employee. Now, what if the employee has worked for 4-5 years and has been assigned a fresh project? The project might complete after a year. Does it mean that the employee should deprive themselves of career advancement opportunities for the year? Therefore, adding a clause beyond the notice period, whether it is valid or not, remains to be seen.
This matter is related to labor but also falls under the purview of contracts. Therefore, while checking with the legal expert, please seek advice from a lawyer who handles cases under the "Indian Contract Act, 1872." Regular lawyers who handle labor law cases may not be sufficient.
Thanks,
Dinesh Divekar