CupONoodles

The Pro B450 and Steel legend is pretty different, the Steel is the premium B450, one of the best B450 out there. For sure has no issues running the new CPUs even overclock. now if you plan overclocking a higher end ryzen 2, probably I would go for a X570.

As in doesn't have a standard 24pin ATX motherboard connector? You can get adapters that convert the 24pin motherboard connector from a standard ATX PSU in to the proprietary connectors (14pin?). So you would be able to plug a new PSU in to the existing system. No need to run two PSUs at once.
They sell for about $2-$3 on eBay from China. Just make sure whichever you buy is compatible with your motherboard.
 
I would probably choose a different PSU if you're planning on running a Vega56. I mean, it'll work... But there's better stuff out there if you're using a high powered GPU.

Yes, but you need to turn both of them on at the same time and bridge the GPU power supply. 
You can also get/make a connector so both PSUs get their POWER_OK (turn on) signal from the motherboard. 

Exactly this. 
Just make sure you bridge the right pins.