Safe to add large external batteries to APC SMT2200RM2u?
First of all, batteries in parallel are much easier to work with. Batteries stacks in series often need a battery management system. The original batteries may have one hidden in there somewhere (could be as simple as some resistors). This is because over time, subtle differences between the batteries will mean that their respective charges might differ wildly (this is assuming you charge the stack of batteries as one unit). In parallel, all the batteries are forced to be at the same voltage, which eliminates the problem. So, either charge the batteries from the stack separately, or periodically make sure they all are charged to the same amount to make sure they don't stop working/catch fire ...

Second, make sure that the voltage of your stack matches whatever it was supposed to be, (Like Jonk says) by charging the batteries before you first install them.

Lastly, you need to make sure that the equivalent series resistance of the new batteries is less than or equal to the original. Given that they are the same chemistry, and of larger capacity you should be good.

And one more thing, lead acid batteries do not like being fully discharged. If you do that once, the batteries storage capacity is ruined.

Disclaimer: Be careful not to shock yourself and you'll have to take full liability for anything that happens.