You can't - simply a limitation of Basic mode and Analog trunks.
Get what we used to call Centrex, where you can flash, sending the caller to hold at the Central Office and give you a new dial tone on the same line, dial another outside number, and disconnect when it starts ringing or after the party answers. Because you are paying for a feature from the phone company, the caller waiting back at the Central Office is now connected to the 3rd party, and your Centrex line is available for another call