There are many insurances that cover pre-existing conditions but to my knowledge they must all be bought within one week to three weeks of deposit to cover pre-existing conditions. There could be an exception out there as there are many insurances. You could always be sure the type of cabin and pricing is still available, then cancel your booking get a refund and then rebook again and purchase insurance within the required time frame.

Most pre-existing condition policies that are paid within the necessary period are also non refundable - some may be transferable but many are not.