I need to raise up my bulkhead door what would be the best way to do this????

It currently sits on a concrete foundation and when it rains alot it comes over the top of the foundation and seeps into the basement between where the bulkhead sits on the foundation. I would like to raise up the bulkhead to eliminate this problem but am worried that a seam between the old concrete and the new would leak. Any ideas on what to do would be welcomed....