is it possible to use a lipsync motion at the same time as webcam?

You can use recorded audio and convert it to lip sync. From the top menu, select Timeline>Compute Lip Sync Take from Scene Audio. It's not simultaneous, but you can go back and disable Lip Sync while using the camera to get just the right head movements. 

Are you talking about the "Compute Head Movement from Speech" option? If so, that doesn't animate your character, it animates the head and based on your rig the rest of the body can take on some movement. If you wanted to start with that you could, then disable the Eyebrow, Parallax, and maybe Head Scale parameters and record a new take of just Head Position and Head Tilt, but that seems like it might cause some conflicts and not necessarily give you what you're looking to do. 

Can you be more specific about what you're looking to do?

You can certainly turn the webcam and audio on and record motion and lipsync at same time. (You can also have a separate audio file and "computer lipsync" from the audio file.)