The Howard League for Penal Reform said the time was added on as punishment for breaking prison rules.
It called the adjudications system, where transgressions are tried, "a monster".
A Prison Service spokeswoman said: "It is right that offenders who break prison rules are properly punished."
A prisoner found guilty at an adjudication can face punishments including solitary confinement or extra days on top of their original term.
Frances Crook, chief executive of the Howard League for Penal Reform, said: "â€ŽThe system of adjudications has become a monster, imposing fearsome punishments when people misbehave often as a result of the dreadful conditions they are subjected to.
"This bureaucratic, costly and time-consuming system of punishments then further feeds pressure on the prisons, creating a vicious cycle of troubled prisons and troubling prisoners."
In Wales' three main prisons - Cardiff, Swansea and Parc - prisoners were collectively given almost 20 extra years on top of the sentences given to them by the courts.
The charity's report described adjudications as "too inflexible to deal sensitively with the needs of vulnerable children and people with mental health problems" and said under the current system, two prisoners breaking the same rule could be given different punishments depending on whether they were on remand or had been sentenced, as well as the category of sentence they had received.
The Prison Service spokeswoman added: "The justice secretary has set out a plan for reform in our prisons, making them places of ambition and endeavour. These crucial reforms will help curb indiscipline and cut reoffending, leading to less crime and safer streets."
Cardiff - 1,229 days
Parc, Bridgend, (G4S) - 4,224 days
Swansea - 1,584 days
Total 7,037 days
Welsh prisoners served more than 7,000 days on top of their sentences last year, a charity has said.