Yes. Jesus died for sinners; His sacrifice was made on behalf of those who needed it — all of us. So the Mass is a re-presentation of that same sacrifice, and every Mass is ultimately no different.
We can offer the Mass for the conversion of all sinners, special persons, whomever. This is why He died; to bring anyone who would accept Him, Eternal Life.
Of course you want the grace of conversion for your parents; and even if your parents have formerly passed, think about the fact that God, who is present to all of time, can see your sacrifice in advance. He can apply that grace from the Mass you offered in time to your parents at their hour of need, when they are facing that decision to accept God's Mercy or to resist Him. If they are already with Him in His Care, He can continue to lavish on them the extraordinary grace and Mercy to sanctify them completely so as to make them fit for Heaven.
We can't fully understand the possibilities of God's reach, but when in doubt, just entrust all whom you love to His Mercy. Masses said are never wasted, and God pours out grace in abundance when we humbly beseech His Mercy.