OK, start out by stripping the main cover insulation about an inch off of each end of the cables. Now strip a half inch of insulation off of each conductor. Twist the conductors from one cable to the next separately trying to keep even tension. You'll have to group some of the 10 pair conductors together and then twist them to a 4 pair conductor. Lay the twisted splices flat and pull slightly to even out the tension. If you have a soldering iron you can solder the joints together for added strength. Then wrap one layer of electrical tape as tight as you can from the edge of the cover insulation across the spliced area to the edge of the cover insulation on the other cable. Then wrap one layer of electrical tape from one inch over the cover insulation across the splice area to one inch over the cover insulation of the other cable.
You now have a strong straight slice about the same thickness as the cable. If you are trying to push pull this through concealed studs you'll still have a problem with the 90 degree bends, especially if the studs are steel. If you have access to an attic, run a new route across the attic and down the walls as needed. Good luck.