Is there a way to fish a cable (network) using an existing one as a guide?
Fish Tape
A real (and &quot;reel&quot;) fish tape is often the tool to use. Because it is flexible and very long (typically 25' or 50'), you can get a lot farther than just fiberglass rods. But it can be a pain to use, depending on the path.
Tape to old, pull new, pull back old
When you have (as I think you do) an existing cable that you want to keep and want to run a new cable in the same path:

Disconnect one end of the old cable
Tape the new cable to the old cable. Any strong tape (both tearing strength and adhesive strength) will do - the stronger the better.
Use the old cable to pull the new cable through to the destination.
Untape the new cable and pull through an additional complete length - i.e., the same length as you had to pull the first time.
Tape the middle of the new cable - which is a location that will become the starting point - to the end of the old cable.
Pull the new cable back down. You will now have the old &amp; new cables in the right place.

If you think you will ever need to do this again - e.g., another parallel cable or a future upgrade - then instead of using one cable to pull another, use pull string:

Disconnect one end of the old cable
Tape the pull string to the old cable. Any strong tape (both tearing strength and adhesive strength) will do - the stronger the better.
Use the old cable to pull the pull string through to the destination.
Untape the pull string and pull through an additional complete length - i.e., the same length as you had to pull the first time.
Tape the middle of the pull string - which is a location that will become the starting point - to the end of the old cable.
Pull the pull string back down. The old cable will now be back in place.
Disconnect the pull string from the old cable.
Attach the end of the new cable to the pull string.
Pull the new cable through to the destination.
Cut the pull string and tie it up in the junction box or other accessible (but hidden is OK) location at both ends.

You now have a pull string that you can use the next time. To use it again, tie a new section of pull string to the old pull string and tape a new cable as well. After pulling through cut off the extra pull string and tie in place for the next time.