High Strength Bars into C-Channels

Is there a piece that allows the high strength piece to go into the C-Channels?

I assume you mean an axle. I’ve heard of teams drilling out holes to go through metal. Or you could build an adapter piece to the low strength, then back to the high strength.

Ya I meant high strength axle, there’s no easier way to do it?

You might want to do something similair to this: https://www.amazon.com/Lock-Hole-VEX-Strength-Shafts/dp/B01MQM1OGD/ref=sr_1_2?ie=UTF8&qid=1542124288&sr=8-2&keywords=vex+robotics+edr

Here’s all of the different sizes of those HS lockbars that are made by VEX Team Virus:
https://www.robosource.net/38-high-strength-lock-bars-1x1-angle

Put a pair of calipers on the 1/4 shaft diagonally. Use that size drill bit. I want to say it is 11/32. Down side is that it is destructive. Make sure you have your hole count right before drilling. Up side is that it is almost good enough of a fit to forego the huge 1/4 shaft bearing flat. (You can actually secure the 1/4 shaft in your drill and spin to round out the hole without shredding off more metal as you would with a bit)(it works especially well in lining up two or more holes).