Will they be added soon?
yes - there’s a pull request ready -https://github.com/diydrones/apm_planner/pull/453
just wait till Bill Bonney accepts it 
until then, you can enter it as “custom” :
ID: 82: MAV_CMD_NAV_SPLINE_WAYPOINT Navigate to MISSION using a spline path.
Mission Param #1 Hold time in decimal seconds. (ignored by fixed wing, time to stay at MISSION for rotary wing)
Mission Param #2 Empty
Mission Param #3 Empty
Mission Param #4 Empty
Mission Param #5 Latitude/X of goal
Mission Param #6 Longitude/Y of goal
Mission Param #7 Altitude/Z of goalCool thanks!