Paul was inspired by God to write his epistles, which are part of the canon.
In the OT, no one understood who the Messiah would be. Paul was a messenger from God, he explained much, especially who Jesus was, and how he came to die for us!
He wrote much that was totally new, as the other inspired writers of the Bible did in their books.
The OT had one major theme. That was to prophecy about God including over 600 prophecies about the Messiah. The law was only a foreshadow of Jesus' first coming.
I preached. Sermon from Gal 3 last Sunday. Here are some of the verses I used, to talk about how going back to the law leaves you a slave to the law! Works do NOT save.
"We ourselves are Jews by birth and not Gentile sinners; 16 yet we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified." Gal 3:15-16
"For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery." Gal 5:1
When you submit yourself to the law, you become a slave to it, Paul says here!
"You are severed from Christ, you who would be justified by the law; you have fallen away from grace. 5 For through the Spirit, by faith, we ourselves eagerly wait for the hope of righteousness." Gal 5:4-5
"For the whole law is fulfilled in one word: “You shall love your neighbor as yourself.” Gal 5:14
"As for you, you were dead in your transgressions and sins, 2 in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient. 3 All of us also lived among them at one time, gratifying the cravings of our flesh[a] and following its desires and thoughts. Like the rest, we were by nature deserving of wrath. 4 But because of his great love for us, God, who is rich in mercy, 5 made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved. 6 And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, 7 in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. 8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast. 10 For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do." Eph 2:1-10
So to answer your question, Blik, Paul neither added nor explained Scripture. Instead, he was inspired by God to write down the words God told him, including that when you try and obey the law, you have thrown yourself into the slavery of the Jews. You you cannot follow the OT, or the law, and Christ at the same time.
Are you for Christ? Or have you been trapped by the bondage of the law!?
You are a Judaiser, just like the people in Galatia were in the church there. Did you know Galatians was the only church Paul did not praise or commend? They were so far from true Christianity, Paul needed to put them straight, and bypass the niceties! Stop trying to convince us to embrace the bondage of the Jewish law in the OT.