The belly hatch in the gondola of the He111 is amply big enough for parachute exits, even employing a static line. The Spanish CASA '111' s used in the BoB movie had been employed in the Spanish Air Force for parachute training, among other duties, without modification. I don't have any information on the particular aircraft used on the above operations, but the transport version would fit the bill, again without any extensive mods. The He111 currently in the RAF Museum, once 'owned' by the 56th FG, 8th USAAF, is actually a transport, and the interior fit is suited to para-dropping operations.
The task of para-dropping agents is slightly different from paratrooping, in as much as this type of operation would normally involve a single parachutist, or a very small group at most, when the interval between jumpers would not be as critical as despatching a stick of paras in the fastest possible time, in order to get that stick onto the DZ safely, without being spread out too far.