It's unsure how the first user of One For All had the ability to to transfer his quirk through DNA. It's not sure how he found out how he had the ability to transfer his quirk. But how he found out that it could be from these three things. He either found out that he could pass on his quirk from either that someone told him or that his brother tried to steal his quirk but failed to do that or when he got the stockpiling quirk.
So if anyone who think that the second user of OFA is Katsuki Bakugo I still find that ridiculous as a theory at all. Because MHA doesn't seem to involve time travel in the story. So I don't know how that the 1st user was able to find out that he could pass on quirks. It's possible that the 1st could find out when he was training the 2nd user. Or before he passed it on to the 2nd user. Or before he even meet the second user.
Now this second part of the theory is that the 1st user found out he could pass on his quirk. When AFO gave his brother the stockpiling quirk. He must of found out that his own quirk was stronger. It's possible that he found out that he could pass on his quirk as soon or after he got the stockpiling quirk. I don't know how to explain that.
Now for the third part of this theory is that when the 1st user got the stockpiling quirk. He might have tried to fight his older brother at some point in time. And I can say that at some point during their first battle. AFO tried to take his younger brother's quirk. But failed on trying to take One For All. I guess that after that one of them or both found out that the younger brother can give his quirk if he willingly allows it. I also guess after the fight the 1st user and All For One. The younger brother decided to pass on his quirk on to future generations. Hopingly that it would grow empower and strong enough to reach to the future and hoping that The Chosen One will defeat his older brother or anyone who has an All For One quirk. And bring a new era of peace.