List is a calculation function, not a script step. The contents of your ToCheckBox field should already be a return-delimited list of the checked items without using the List function. (The order of items in the field will vary depending on the order the items were selected.)
It's not clear what you're trying to accomplish. What field(s) are you importing from? What field(s) are you importing to? Do you want to create separate records for each selected value, or each value in the value list regardless of whether or not it's selected? Do you want to set selections in the ToCheckBox field based on an import from multiple other records? What's the workflow that this import will be a part of?
If you are exporting the data to a text file for import by another application, the returns entered into the field when you select multiple check box values will be replaced with a different character in order to keep the data inside a single row (record) of data.
You may want to use a portal to a related table instead of a check box field for this data so that you can export the data as separate records.
See the "check boxes with scroll bars" examples in "Adventures in FileMaking #2 - Enhanced Value Selection" for some ways that you might set that up so that you still have your check box functionality but generate records in a related table.