But the Borg's war of adaptation is more one of forcing a way; changing the frequency of your shields ans weapons to penetrate others and at the same time preventing enemy fire from penetrating their own shields. Creating for example the Section 31 virus is impossible for the Borg, a process of trial and error won't do them any good here. The only thing they can do is assimilate some one, aka a human, who knows the formula of the virus.
The Borg are unable to create new technology, the only thing they can do is assimilate it.
The borg adaptation technology is FAR MORE than merely changing the frequency of their shields and weapons.
You see, the borg adapt NOT only to weapons, but to ANYTHING - tactics, strategies, attempts to hack the hive ming, to disable nanoprobes or other components, etc.
About weapons - energy weapons are FAR TOO COMPLEX (to be exhaustively described by "frequency" and adapted to by merely changing the frequency) and FAR TOO DIVERSE (phasers, disruptors, plasma weapons. matter-antimatter warheads, quantum torpedos - and I'm not even mentioning the more exotic ones).
And the Borg can adapt to them ALL - at least, all we saw on-screen - except 8472's, but those weapons were out of this world/the trekverse (literally) - plus, you can be sure 8472 did some adapting of its own.
The borg adaptation technology works on the same principles as darwinian evolution - through trial and error.
If a slightly better (more adapted) version is found (after trying a large numer of versions AKA through trial and error), it is retained and improved (through the same process).
Darwinian evolution used this process to make life - and us (extremely complex creations).
The borg use a similar process to adapt.
The difference is - darwinian evolution needed BILLIONS of years to do its job.
The borg only need SECONDS to adapt - as I said, a TRULY MAGICAL SPEED.
It is a truly remarcable technology - second to none (which includes creative federation engineers) and better than most (when it comes to weaponry, it proved again and again to surpass by far the likes of Geordi or B'Ellana).
In order to assimilate a founder, all the borg have to do is try to assimilate one with their nanoprobes. From the BILLIONS of nanoprobes used (all trying something else), some will behave a little better than the rest. The borg will use these second generation nanoprobes and repeat the process. In a short time, the borg will obtain nanoprobes effective against the founders' "anatomy" - much like S31's virus was.