A couple years ago I tried using a Futaba wall charger (4.8v - 100mah) for a 6v-1700mah pack. Didnt work well at all. Just wasnt enough output for an overnight charge and it never would peak the pack.
My impression is that a lesser capacity 6v battery would have probably worked ok. Thou I havnt tried it since at that time I was switching to mostly 6v Rx packs and went ahead and bought a nice high end charger.

Theres no way Id risk charging a 6v battery from the 9.6v side of a wallwart. As CGr said, 6v packs normally peak around 7v.

Cheers.