Check your feeder size
That feeder coming off those 100A breakers needs to be either #3 copper or #1 aluminum. If it is #2Al / #4Cu, then you must downgrade the 100A breakers to 90A. Those are the rules, I don't write them. If you were misinformed that #2/4 is good for 100A, that happens a lot because people love to misinterpret 310.15(B)(7), or blindly "take on faith" answers from others who did so.
If conduit fill allows, lean toward aluminum wire. It's always worked fine on large feeders like this.
Accommodating that extra conduit
No need to crack the wireway issue. We can do this another way: install a splice box within 2' of the panel, and fit maximum conduit size between splice box and panel.
Pick either the feeder/conduit on the left, or the feeder/conduit on the right.
Pull out the existing feeder Note the concentric knockout that it comes in, is not at its maximum available size. Break off the concentrics so it is at its maximum size (2-1/2"??).
Install a new large metal junction box below the service panel. The top hole gets a hole in the top the same size as the knockout you just opened up. The bottom gets a hole fit for attaching the existing conduit (1-1/4"?)
Install an EMT or Rigid metal conduit nipple that is less than 24" long between the service panel and this new box.
The existing conduit will be in the way of the new box. Cut the existing conduit out of the way, and apply fittings so it now enters the new box.
See what we did there? You now have a 2-1/2" (?) pipe from the service panel to new box, and a 1-1/4" (?) pipe going wherever that conduit currently goes.
Now, pull the wires back into it. We're back to status quo ante.
Now, bring your NEW conduit into the side of the new box. Pass its wires through the box and up to the service panel through that nice fat conduit. Voilà!
If other conduits are in the way, like that 1/2" conduit, bring them into the new splice box also. More's the merrier!