You need a field to copy the data into first.
If you define new fields in the target table first, you can then use Import Records with the update matching option to update your existing records with the new data.
You could also link these two tables in a relationship (You can link to tables in other FileMaker Files) and then use a script or Replace Field Contents to pull the data from a field in one table into a field in the other.
Thanks for the quick reply.
I found that your suggestion indeed did work.
I added the fields I needed into that database and then did an import with a matching option.