27" Apple Cinema Display (video and power) to one USB-C I've got an Apple Cinema Display with ThunderBolt-2 and MagSafe-1 connector and would love to combine these two cables to one ThunderBolt-3 to supply power to the Mac and video and sound to the display on one cable.

The TB-2 on the outside is connected to a TB-2 port on the inside.
The MagSafe-1 disconnected to two "pins" GND and Voltage on the inside.

There are TB-2 to TB-3 adapters (e.g. Apple) on the market. Is there a way to connect the "power pins" to the TB2-3-adapter and solve my problem?

(I've done a lot of soldering and working with Arduino etc.)
Is there a way to connect the "power pins" to the TB2-3-adapter and
  solve my problem?


No.  This can't be done.

This is not a matter of just dumping in some power on some of the pins.  Thunderbolt 3 supplies USB 3.1 signaling which conforms to the power delivery specification.  Meaning, power is negotiated between the two devices for up to 100W (20V at 5A); it's not simply supplying a set voltage (5V) at a maximum amperage of 3A (USB 2.0 spec).  



From the image above VBUS (power) is provided on pins A4, A9, B4, B9.  However, it's not a constant, it's negotiated over the CC bus (CC1 and CC2).  By hard soldering in the power (16 to 20V DC) from the Magsafe, you could potentially fry equipment that's not expecting (or requesting) that much voltage.