User exit for VL01

Hi can some body tell me which user exit we can use for VL01N ( It seems that there is no Customer exit for this application) my requirement is I need to update one custom field( in the table LIPS(Delivery line item) the custom field is contract number 'Z_Contract'(The type of this field is VBELN) which I have added to the table LIPS using append structure.

Please let me know how to populate the field data of the table LIPS in which user exit we can do this?

Thanks&Regards

Mahesh