Why do you want to do this - the pendant's case can't have room inside for 2 x AA batteries so you'll end up with a much bigger device - with an external supply it would have wires as well.
You should assume that it needs the correct supply voltage of 3V.
For something safety critical like a lifeline alert you should not modify it unless you know very well what you are doing and also have the facilities to test the modified device properly.
Kind of a newbie at this, so forgive me if this is a stupid question, but if I've got a small lifeline alert pendant transmitter that normally runs on a CR1632, can I replace it with two AA's? Are there any disadvantages? Do CR batteries last longer, have more capacity than AA's?
On a separate note, if I want to replace the batteries with a power supply, can I use a 5v USB supply or would I have to look for a 3v supply?