There is nothing to prevent you from taking insurance policies apart from ESI coverage. Only one point you will have to consider is that to claim insurance money you will need to submit the original documents. Your predicament would be as to how to submit the ESI documents to the insurance and claim- if the employee has gone for ESI for medical benefit and also cash benefit.
Alternately if you want to opt for the insurance coverage and not opt for esi though contributions would be paid, then ensure that the company's interest is safeguarded as by ESI coverage the employer is indemnified from all claims under the Workmen's Compensation Act, The Maternity Benefit Act and the Fatal Accidents act, apart from affording medical treatment to the employee. I do not suppose there is any policy which can replace the ESI scheme by way of benefits - quantum & variety. Please check all aspects before doing anything in the matter- as once committed it should not only be affordable and useful on the long run.
12th March 2010 From India, Bangalore