A wizard did it.
IIRC, Adeen is living in a nunnery somewhere in Issac at the start of chapter 6. I think Lester says something to Rana about it.
As for the other characters between FE7 and FE6...well, I'll start I guess. I'll use what we know.
We know Lucias started an orphanage. Chad/Lugh (can't remember who) says that father was killed by the Bern soldiers. Since they lived at the orphanage, it wouldn't be that much of a stretch to assume that Lucias was killed by Bern, would it?
Canas and his wife spawn Hugh, and then they die trying to stop a snowstorm. Niime raises Hugh.
Rebecca spawns Wolt. IIRC, she feeds the infant Roy and infant Wolt. Or it was somebody else who fed them.
Wallace probably got lost somewhere.