Hi Bob,
We faced a similar situation with our system some years back. The existng bank (purchased with our initial system) simply wasn't big enough, and buying all new batteries just was not in the budget at the time. Plus, it just seemd like a waste to me. In our case the first batteries suffered more from inactivity then old age, so we had to work on them a bit. It is probably quite dated now, but a fellow named Michael Hackelman wrote some useful strategies on testing, caring for, and in some cases rescuing [partially] sulfated batteries. Anyhow, I think it worked out okay overall.
In your case, at 1-year old, your batteries are relatively new and probably just hitting their stride. Generally they're a little stiff when new, so I wouldn't hesitate to add another string - of the same make/capacity is even better still - to your bank. Just make sure the connecting cables are adequately sized.
Most manufacturers will tout their (improved, superior) plate construction, DOD rates and life expectancy under very controlled conditions, and rarely are real-life conditions ideal. Having said that, lead-acids are pretty resilient.
We're now replacing our existing bank, after almost ten years. The truck is scheduled to be here friday with the new ones. We'll have two parallel strings (48V), and I'm giving serious consideration to replacing a single series-string at their ~mid-life. The alternative is very high cost at end of life, and dismal performance in the final stages.
One final thought. We've have, and have used, both the T105's and the L16HC's. I ran some cost comparions based on $/AH nsome years ago. The T105's actually came out on top as I recall, but the life expectancy is much lower (4-7 vs 7-11 years). We had good performance and reliability with both.
kind regards,
-kitestrings