Importing entries from one termbase into another one with different termbase definitions

We created a brand new termbase for our team with MT 2021. We are now trying to import terms from our legacy termbase (also MT 2021) into the new, empty termbase. However, the termbase definitions of the two termbases are different.  How can I still get the data into the new termbase? So far, I have not been able to import a single term. Is there now way to import the fields that match and to leave the other fields blank so that we can edit them later? Sure there must be any other way than manually adding all the terms again.