When a second call is coming in, you can directly initiate a conference by drag'n'drop the call from the Incoming Call list to the middle of the screen.

This can be done with every additional call which is coming in!

Direct conference

If you have a call established and you want to call a 3rd party into this call, open the dialpad, press the green receiver-symbol (with +), enter the phone number and start dialing. The call will be established directly as a conference.

Conference with callback

Another way to establish a conference from an active call is to put the first caller on hold, open the dialpad, press the green receiver (with +), enter the phone number, start dialing, wait for active call and then drag'n'drop the call on hold to the conference-option.