Correct. It's impossible to add to Jesus' work on the cross, nor His resurrection from the dead. Once He was bodily dead, what else can men do that crucified Him? Much less anyone who lives after His bodily death.
Luk 12:4
And I say unto you my friends, Be not afraid of them that kill the body, and after that have no more that they can do.
Nor of course, can any person on earth add to His resurrection from the dead. Which God alone can do:
Rom 10:9
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.
And so, repentance and confession cannot be adding to Jesus' sacrificial work and resurrection. Rather repenting for forgiveness of sin and confessing Him Lord, is from His sacrifice and with His resurrection, not adding to it.
In fact, such repentance and confession unto salvation is also given by God, and not a work of men alone:
Act 5:31
Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins.
Acts{11:18}
When they heard these things, they held their peace, and glorified God, saying, Then hath God also to the Gentiles granted repentance unto life.
And though we must add good works to our faith with our salvation, for a sure election and entrance into Christ's eternal kingdom, once again that's not adding anything to Jesus' work of sacrifice on the cross and resurrection from the dead.
2 pet 1:4
And beside faith, giving all diligence, add to your faith virtue; and to virtue knowledge; And to knowledge temperance; and to temperance patience; and to patience godliness; And to godliness brotherly kindness; and to brotherly kindness charity...
Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall: For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Saviour Jesus Christ.
As you say, such works of faith by repented confessing believers, is only a necessary part of our own salvation in this life, which can only be done
after Jesus' work on the cross and rising from the grave.
Phl 2:12
Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
And so, while we must repent and confess to be forgiven and saved by His work of sacrifice and resurrection, we do not do so to add to His work, that only He can do, but only to partake of His work in saving us from our sins:
Mat 1:21
And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins.
2 Peter 1:3
Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
Rev 20:5
Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.