E-chits - Payment Type / Reference

It is not currently possible to add to the Payment Type / Reference drop-down list on an e-chit.
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The e-chit - Payment Type / Reference drop downs contain selections of either CHQRUN/BACS/CHAPS, which once selected assigns the posting (Once posted via Admin Tasks) to the Payment Processing module, where they can be picked up for Cheque Printing/BACS/CHAPS Uploads.
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However, this field can be free typed into. This then ignores sending to payment processing and assigns the Payment Type / Reference to the ledger posting only.