Answer: Head to admin>config>destination school setup. Once in here click "add new" and you can enter the relevant details. You are likely to need to use XXX in the destination LEA and ESTAB numbers due to either not knowing the numbers or simply the differences across national school systems. For "Destination School Name" you may enter something of relevance, such as the country destination or "Moved abroad" - the choice is up to you. For schools in parts of the UK not in the English education system you can enter the school name as a record for their profile.
You can then record this on the Extended tab of the student profile, under School History. This means their destination school will be recorded like the rest of the students and can be reported on in the same way.