No. The session is built between endpoints.. and once a "call" is originated/answered by a set registered to an extension, the other handset/phone cannot "pick up" the "line" and "listen in" or "add video".
More of a function of SIP. That being said.. the protocol itself may have such an option.. but I don't think it is implemented with 3CX to do so.
We had somebody ask once, and looked into the same thing. You have a call in session on a handset, and want to "add video" on a webcam via the softphone.. no.