I wrote a simple user migration of user data (email, status, Name) which is working. I also have a custom field (field_ipaddress) added to the account but I am having difficulty migrating to it.
I tried adding the code to the main migration which makes use of an entity: user destination and a separate migration after the main migration which is using the destination of entity:field_config.
The module maintainer has gone MIA and so it was picked up by another dev and stored on Github here. https://github.com/bjaxelsen/field_ipaddress