Is it possible to split micro USB to USB cable so two cables (data + and data -) go to the PC with USB and two other power cables go to power brick?
Yes, but not quite what you suggest.
You would have to feed D+/D-/GND to the PC, not just D+/D-. This would allow the PC to communicate without supplying power.
It's then possible to split off the +5V cable and GND to an external +5V power supply. In order to do this, you would need to split open the cable where you want to splice off your power supply, preferably without damaging the D+/D- cables as these are controlled impedance and don't particularly like having random bits of cable or solder joints in them.
Once the outer insulation is split open, you can remove a small section of insulation from the GND cable and solder on the GND from your second cable. Then also cut the +5V cable and solder it instead to your second cable.
Finally, carefully cover everything up with heat shrink tubing or similar.
Basically you would have something like: