The botlab items cannot be simply added. They are instantiated with random stats and separate IDs from template items.
In theory you could do it with (example for Gen 3 legs) GAME.lab.parts.append(GAME.sbPartsData["SBPLegs3"].clone())
The template item codes you can find in one of the sexbot_lab rpy files.
One typo though and you might nuke all you Botlab Parts or corrupt your game so i recommend against it unless you really know what you are doing and have some python basic syntactical knowledge.