You can not pay. If you continue to work after the age of entitlement to state pension, you do not pay NI.
If you are under the age of state pension entitlement you do pay NI regardless of how many years you have paid contributions.
"Under pension age
You have to pay national insurance contributions while you are working and under pension age, if you earn more than a certain amount called the primary threshold. You do not pay them if you are under 16, and you stop paying them when you are 60 (if you are a woman) or 65 (if you are a man). If you are a man between 60 and 65 and you are not working, you may be able to get national insurance credits (see under National insurance credits)."
http://www.adviceguid...ions_and_benefits.htm