Don't export, import. You can import data from one table into another table in the same file via the Import Records menu option.
When you do, you'll get a small funky dialog that pops up as the last step with an option that enables you to split the repeating data into separate records.
So you specify the unique ID field and each of the repeating fields as fields for the import and then select that option. If you use the "new table" option for the target table, you can even create the new related items table as part of the import.
a couple good tips:
"Splitting FileMaker Repeating Fields"
"Converting repeating fields to portals"
Also these functions may help:
Extend, GetRepetition, and Last
In addition there are other functions that work with Repeating fields.