Hi, if there is an existing routine, you can make the changes in the same one. Routine number 900 onward is for customer modification. If there is none assigned, you can assign one which has number between 900-999.
Requirement routines are created in Transaction VOFM-Requirements-Output Control. ABAPer needs to do this. Assignment is done in NACE-Access sequence, this is done by functional consultant.
There is a wiki here for your reference:
http://wiki.sdn.sap.com/wiki/pages/viewpage.action?pageId=33751376&bc=true