Each fare is different, some allows 1 free stopover, some allows 0, some allows more, so without knowing the exact fare rule we can't say if it is correct or not
To do such booking online, you can try the multi-city booking tool with 3 segments (LAX-CPT, JNB-CDG, CDG-LAX 3/4 days later), if that doesn't work then your only option is via the phone or social media, if it cannot be booked online then they should be able to waive the agent booking fee.