If it's twisted pair, you can only try. If the extension cable is untwisted, forget it - it's so far out of specification as to be a waste of time.
An old installation trick to upgrade a cable that runs in duct or trunking without significant bends is to carefully wind the wires of the old and new cables together, sleeve the whole lot in tape and use the old cable as a draw cord to pull the new cable through the duct. You then cut off and throw away the last metre of the new cable when it arrives at the other end, as it's usually pretty mashed.
This can be a life saver, or a disaster if the two cables come apart mid pull. Obviously, it won't work if the cable is stapled or clipped for any part of its run.