Adding a new lead acid battery in parallel to an old one?

In the end, it will not cause a safety hazard--Just an unknown interaction between old an new deep cycle batteries. You may end up having to replace the older battery in 3 more years--And it will be unclear if the new battery will last another 6 years, or age faster and die around the time of the older battery.
If you need the extra capacity (and have the solar/genset/charging resources to support a larger battery bank)--It may still be worth it for you to install the new battery and see what happens.
The downside being that the new battery may not last as long or you may get in the cycle of replacing a failing battery every three years instead of both batteries every 6 years (nominal numbers, your actual battery life will probably be different).
With any parallel battery bank setup (and for general solar power system debugging in general), I would highly suggest getting a DC Current Clamp Meter (Sears sells one with DMM funtions that is "good enough" for ~$60).
Once or twice a month, go out to the battery bank and when under load and under charge measure the current into each battery (string) and make sure they are (roughly) sharing the load/charging currents.
When on battery stops "sharing" the current--You need to debug (look for open/shorted cells, as well as bad electrical connections).
I am one of the first people to try and guide people away from mixing batteries--But if you are up for the added maintenance and want to give it a try--Why not.
If this was a "critical" power system--I would highly recommend not mixing battery banks. However, if you can spend the time watching the system and replacing one battery at a time--go ahead.
Note, when you parallel batteries for signal station, you should have a fuse/breaker per string to prevent a short on one battery string from being feed by the other string--this does add wiring/costs to parallel battery system--and one of the many reasons why I/we really recommend going to a single string of larger AH batteries rather than paralleling--others include more electrolyte caps to check, more cabling to check, more cells/batteries where something can go wrong, hidden failures that can damage other batteries like shorted/open cells, etc.