It is possible, but not recommended to change sap standard. You will have to get a Key from sap frist in order to change this, and then get an abapper to do this, but i would investigate other alternatives rather than changing standard code.
Maybe you can copy the standard program to a Z* program and then change it, or maybe there is an older version of this program in the version management of this standard program which you can dig up and create a Z* program from this? All depends what is there in previous versions.