You can't convert, you just have to meet the minimums and apply. They also don't have class ratings in quite the same way: you can get an ATP ASEL, AMEL, ASES or AMES. (Airplane Single/Multi Engine Land/Sea) you have to take one of those tests as an initial, generally AMEL.
Before you can take the AMEL written test you need to complete an ATP-CTP simulator course, then take the written, then take the check ride.