What is provided is a method of adding a functionality to an executable first module of a program package, wherein the executable first module, during execution, provides an internal function or utilizes an internal function of an executable second module of the program package; wherein, when the first module is not executed, an externally visible reference pointing to said internal function is determined, auxiliary code is added to the first module and a redirection is permanently provided in the first module as a function of the reference thus determined such that a modified executable first module is present, wherein, when the internal function is requested during execution, a redirection is made to the auxiliary code which causes the functionality to be provided during execution.