I just paralleled a new set of batteries with my original set that have about 80 full cycles through them. These are larger Li NMC cells, but the idea is similar. You do need to get them as close as possible to the same state of charge. As was suggested, using a separate charger to bring each one to full is a good idea. That should have them all vey close, but check the resting voltage before you connect the cables. Even 1/10th of a volt can mean a lot of current between the batteries. In my case, my new battery was charged higher than I normally run my system, so I ended up using a DC-DC buck converter to pull some power from the new battery and push it into the older battery which was still running my system. It took 12 hours to bring the new battery down to match the old one. I then paralleled them first through an 4 ohm resistor and measured the voltage across the resistor. This showed that I was well within 0.01 volt. So I then connected the cable direct and checked again with a DC clamp meter and saw less than 1 amp, so I was good to go. They have now been tied together and cycling for over a month.
The cables to the new battery are longer, and I used a cheaper BMS solution. I fully expected the old battery to carry a little more current. I was wrong. The newer cells with virtually no cycles apparently do have lower internal resistance. When I am charging at 30 amps, the old cells are taking 13.5 amps and the new pack is taking 16.5 amps. On discharge, I see the same thing as well. When the packs rest, they are hitting the same resting voltage and the current drops to true zero with no flow between the two packs. So the newer bank is moving about 10%-15% more watt hours than the older batteries to hit the same state of charge levels. So the new batteries are working harder, for now. Yes, it will age a little faster, but what should happen is the true internal cycle life will basically just "catch up" to the old bank and the current will end up balanced again at some point. This is not a big deal as I am pulling far less current than either bank could take on it's own. This would only become an issue if you wanted to pull 300 amps from 3 separate 100 amp rated batteries. That is a bad idea as they will never share he current perfectly. Adding another battery to get more run time is fine, but don't ask for more maximum current.
With my doubled up battery, I went from cycling 7.5 KWH a day to cycling 10 KWH per day. So both batteries are still cycling quite a bit less than what the one original battery was doing. I am only using less than 30% of my battery capacity on daily cycles. This should greatly extend the life of the cells. And I have a ton of reserve if I have a grid power failure.