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.
Yes, the source code is needed for such a change. In short, (overly simplifying) everything in any game is written, scripted or programmed. AI behavior is governed by scripts in WFaS, therefore one has to locate and implement said scripts into one's own module. The source code of WFaS is publicly available, but without the scripts handling what you are seeking, you will not achieve your ultimate goal.