I believe your initial solution looks good, I would just use map_reduce instead of reduce to clean things up:
reservations = [%{rooms: 1},
%{rooms: 2},
%{rooms: 3}]
{reservations_with_room_numbers, _} =
Enum.map_reduce(reservations, 0, fn reservation, offset ->
upper_limit = offset+reservation.rooms - 1
room_numbers = Enum.to_list(offset..upper_limit)
reservation_with_room_numbers = Map.put(reservation, :room_numbers, room_numbers)
{reservation_with_room_numbers, upper_limit + 1}
end)
for 
