Hello,
The current version of modifyInputFields doesn't include a parseLineBreaks option to add contents with line breaks (other methods such as replaceVariableByText or replaceTableVariable include it). We have added a task to be added in a future release of phpdocx.
If you are interested on a custom development of this specific task, please purchase a pack of 8 tickets (https://www.phpdocx.com/support) and open a ticket with this same request. The dev team will develop this feature and send you the updated class with a sample.
Regards.