SSCC barcodes are GS1-128 barcodes with the Application Identifier (00). SSCCs are 18 digit numbers (17 digits + check digit). ITF 14 is a 14 digit number. This limits you to encode ITF14 directly into the SSCC. You can add the trailing zeroes until the 17th digit and let NiceLabel calculate the check digit, but the recipient must also be notified about the change.
If adding zeroes to the end is not an option and you do not want to encode ITF14 into SSCC, you can use some other AI. Both sender and recipient must agree upon the GS1-128 barcode format regarding the selected AI.
Here is the list of Application Identifiers: www.gs1-128.info