Campaign Missions differ from free roam in that they are intended by the author to have a certain weather. It would make little sense for the player to change that.
Free roams are constructed in the same way as missions—they just have an environment and ships, but no objectives and no logic except for that needed to support any AI.
You can always make your own free roam in the Mission Editor. You can then include additional Player ships, moorings, barges, etc. You can set whatever dynamic weather you wish, and you can even have it change when you enter different areas of the environment.