I've read a number of forums that recommended Zerex G-05 as a substitute for Volvo's coolant, after reading a few forums, I'm confused as to whether it's safe to just pour directly into the car(after mixing it with water), or do I have to drain it...which I don't know how to do.

I have a used 2000 s80

 

Have you read this.

http://www.valvoline.com/pdf/zerex_g05.pdf

Basically you are adding a HOAT type antifreeze to your existing system????
You need to know that the existing is also a HOAT. It should be.

I use a NULON Long Life HOAT type. 50/50 mix.
Green in colour.
The spec's for that state never to mix a green and red type together.
Low silicate, low ph, phoshates free.

Their (Nulon's) red type is a OAT as opposed to a hybrid.

 

If they're both HOAT can you mix them?

 

If they are both ethyl glycol based and HOAT then probably okay.
Personally I would not mix it unless the existing coolant was relatively fresh.
If older, I would use the situation as an opportunity to renew the coolant complete for piece of mind and good maintenance practice.