Yes, they stack. It says so in the Frenzy class ability description. However, if you Rage and Frenzy at the same time, you are Exhausted rather than Fatigued when they both run out.
That is just brutal....the NPC i am creating will have a +10 to his strength with the frenzy rage stacked. And considering the improved power attack (+3 for every 1 point penalty when weilding a a 2 handed weapon), die hard feat and deathless frenzy the PrC seems a bit over the top. But i do have to say that that is one angry bad ass Orog beserker
Half-ogre/half-dragon (or go Dragon Disciple if you want more hitpoints, but you'll lose Rage progression) barbarian / FB. Enjoy str+24 without magical boosts or perhaps more for better rages. Perhaps some racial half-dragon paragon levels boost your strength even more? Or what about the warhulk?
That's the whole point. It's very nasty. And it's just as nasty when it turns on your allies, who, unless you can make that save to snap out of it, are seen as enemies once you run out of actual enemies.
...and since they're seen as enemies, no wussing out and pulling your punches. Nothing bugs me more, personally speaking, than a FB that does everything in their power to flub their own attack rolls once they turn on their friends. Full power attack, fighting defensivly, using non-proficient weapons and so on.
You want the power, you pay the price, and no skiving out of it.