Yes, it is true! You can!
The adorable Hazelnuttygames has made a simple video guide on how to do this!
I post it here, because I´m one of those, newest of beginners when it comes to Pet Battles. I often find I have little time for it (lol, priorities, right) and when I finally want to do it, I get discouraged because I miss this or that Pet, whatever…
And while you go about doing your daily things, World Quests and the likes, you will loot a Pile of Pet Goodies. This will grant you those Pet Charms and for those you can purchase the Ultimate Battle-Training Stone, which levels a pet up to max! (Bought at Lio the Lioness)
I had no idea about all of this, hah. Maybe now I can finally get started.
Hazel also has a great guide on which pets will help you in Legion.
Hope it´ll help you as it has helped me!