I have a health-insurance policy of X company but they do not offer top-up plans. What should I do?

You can buy the top-up plan and the health-insurance plan from two different companies, but it is advisable to buy these from a single insurer as it will save you money and time. If your basic policy and the top-up policy are from two different companies, informing two insurers and getting the claim settled may be cumbersome.