Acts 6:8 - Stephen is described as "full of grace" (πλήρης χάριτος) In Acts 6:8, according to some manuscripts the Greek text states,


  Στέφανος δὲ πλήρης χάριτος καὶ δυνάμεως ἐποίει τέρατα καὶ σημεῖα μεγάλα ἐν τῷ λαῷ. (NA28)


which is translated by the NABRE into English as,


  Now Stephen, filled with grace and power, was working great wonders and signs among the people.


The Greek πλήρης χάριτος literally means "full of grace" (cp. John 1:14). In the Vulgate, St. Jerome translated that Greek phrase into Latin as plenus gratia, the equivalent of gratia plena in Luke 1:28, the only exception being plenus declined in the masculine gender referring to Stephen and plena being declined in the feminine gender referring to Mary (and the word order which is inconsequential).

That being said, it is reasoned by Catholics that the virgin Mary was sinless because she is gratia plena, "full of grace." Can it be also said of Stephen that he, like Mary, was born without original sin and that he also never sinned, since he too is described as being plenus gratia? Why or why not?
In case of Mary, the angel refers to her, as you have said, as one who has been filled with grace, and the church teaches that she received a special grace at her conception which allowed her to be born free of the effects of original sin. This grace did not preserve her from sin, per se, but this grace made it possible and so she continued in her cooperation with the will of God throughout her life. The infusion of grace she received was an application of the grace won by Christ on the cross, as Christ's merits are eternal and are not restricted only to points in time after his crucifixion and resurrection.

In the case of Stephen, the church does not claim that he was sinless in its doctrine. You are correct, however, that the phrase is referring to the same phenomenon: a radical infilling of grace. Because Stephen was a deacon, we can be sure that he would have been a baptized member of he church, as Christ commanded in Matthew 28:19 and was full of the Holy Spirit. Being full of the Holy Spirit is one of the apostles' criteria for selecting deacons (Acts 6:3). Catholics attribute Stephen's having been filled with grace to his baptism, since baptism confers grace, remits all past sin, regenerates us, cleanses us from original sin, and causes us to be born again of water and the Spirit.

So while Mary was free from the effects of original sin at every point in her life, tephen, unlike Mary, was not free from the effects of original sin prior to his baptism. In other words, there was no point in Mary's life where her ability to remain obedient to God was impaired by the effects of original sin. Moreover, Mary was sinless because she continued to say yes to the will of God throughout her life, and her unswerving obedience gave her the freedom and ability to obey. 

Likewise, the baptized person today is freed from sin and put into a new relationship with God through the grace of baptism, as Romans 6:4:7 states: 


  Therefore we have been buried with Him through baptism into death, so
  that as Christ was raised from the dead through the glory of the
  Father, so we too might walk in newness of life. For if we have become
  united with Him in the likeness of His death, certainly we shall also
  be in the likeness of His resurrection, knowing this, that our old
  self was crucified with Him, in order that our body of sin might be
  done away with, so that we would no longer be slaves to sin; for he
  who has died is freed from sin.


... And verse 14 states:


  For sin shall not be master over you, for you are not under law but
  under grace.


In conclusion, Stephen's being full of grace (πλήρης χάριτος, which literally means “full of grace”) was not the same as Mary's being full of grace (κεχαριτωμήνη--kecharitomene, which means “she who has been graced”). This slight difference in the two expressions indicates that Mary never sinned but that Stephen, unlike Mary, was not free from sin.