is it possible to place a marsh on an opponent's group which is at R0 during the game (not in the initial terrain placement phase)?
Except at the beginning of the game, during initial terrain placement, NO terrain card be placed on ANY group, friend or foe, unless that group has a Movement card previously played on it. (BTW, There can pinned men in moving group.) Range makes no difference.
(Wire and Smoke are NOT terrain cards--they modify existing terrain cards and thus can be played on non-moving groups.)
BTW, playing any card on your opponent: Terrain, Wire, Smoke (though this requires a US or German AFV with ordnance effect of 4+) or Sniper is accomplished with a discard.