Certainly Jesus suffering was sufficient for the whole world, but it is our playing our part to spread the benefits of that suffering…through prayer, almsgiving, works, suffering, etc.
Jesus says:
(Luke 14:27)
Whoever does not carry his own cross and come after me cannot be my disciple.
St. Paul spoke of this many times:
Colossians 1:24
Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in the afflictions of Christ on behalf of his body, which is the church
2 Thessalonians 1:4-5
4 Accordingly, we ourselves boast of you in the churches of God regarding your endurance and faith in all your persecutions and the afflictions you endure. 5 This is evidence of the just judgment of God, so that you may be considered worthy of the kingdom of God for which you are suffering.
Hebrews 12:11
At the time, all discipline seems a cause not for joy but for pain, yet later it brings the peaceful fruit of righteousness to those who are trained by it.
I could go on and on with examples…
It is not that penance makes up for anything lacking Christ did on the cross, penance (in the various forms it comes in), is our willingness to follow Christ.
Penance strengthens us spiritually and can be used as prayer (fasting, for example).
The life of a Christian is a life of penance. (I forget which Saint said this.)
In summation:
It is what we do here on earth that continues the purpose of the crucifixion of our Lord. Our works (cooperating with His Grace) are what make salvation possible for others. If no one chose to follow Christ, no one could be saved.
We do penance to share in Christ’s suffering. We do penance to commit ourselves more deeply to God.
We are not adding to the cross, per se, we are continuing the cross.