Add what to what? Be descriptive, because your thread does not include solid information and is vague.
Firstly, state what modification does not have the skirmish option implemented. Moreover, check whether its source code is available to the public. Secondly, state what module has the above-mentioned option implemented. Do the same with respect to its source code. Thirdly, if you succeed in obtaining the code of both modules, all you have to do is merge the option with the whole of the module of your choice.