It doesn't matter what the rated voltage output of the charger is. As long as you are physically able to push current through the battery, then it will eventually recharge. Wall-warts will definitely recharge your 5 cell packs, fully, unless they are particularly weak. Most of the ones that come with the name brand radios are sufficient to recharge virtually any RX pack, regardless of voltage. To say that a charger cannot charge a battery past it's rated output is WRONG. Car battery chargers have rated outputs of 12 V, yet are plenty capable of charging the batteries to up over 14V.
Also, if a charger could only charge to it's rated output, then there would be no reason to take a battery off of a charger. Take your 5.8 V walwart for example. If that is all it is capable of taking a battery to, then why worry about how long a 4.8V battery has been on the charger? A 4.8 V battery is plenty cabable of safely handling 5.8 V, so why not just leave the batttery on the charger all week or month long, and just take it off when you are ready to fly? The answer is because if left on long enough, you will put more than 5.8 V of potential back into the battery, which can cause irreversible damage to the pack.
Remember, a charger uses CURRENT to reverse a chemical reaction that took place when the battery was used. It does NOT use voltage. The voltage in the battery pack will go up in response to the reversing of the chemical reaction. Where voltage in a charger comes in is in the fact that the more voltage a charger has, the more current it can push through a battery to more quickly re-charge it.
No, I do not believe that you could use a wal-wart to recharge a 6 or more cell pack, as the voltage potential in the charger isn't enough to coninuously push current through the pack as the pack's voltage increases. However, when we are talking about 4 or 5 cell packs, then there is no problem at all......just give it enough time.
The first time, I would charge it for 20 hours, and then test the battery with a multimeter. Then charge more as you see fit. Keep testing it every 5 hours or so, until you are satisifed that it is fully charged. You will only have to do this once. After that, you will know how long it takes...(of course, this all depends upon how discharged it was to begin with).