No, it is not permissable to change the words of the Sign of the Cross from the approved translation in the Roman Missal.
From the 2004 Instruction Redemptionis Sacramentum:
“[59.] The reprobated practice by which Priests, Deacons or the faithful here and there alter or vary at will the texts of the Sacred Liturgy that they are charged to pronounce, must cease. For in doing thus, they render the celebration of the Sacred Liturgy unstable, and not infrequently distort the authentic meaning of the Liturgy.”
I think a commentator can say things before the Sign of the Cross. From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“105. … b. The commentator, who provides the faithful, when appropriate, with brief explanations and commentaries with the purpose of
introducing them to the celebration and preparing them to understand it better. The commentator’s remarks must be meticulously prepared and clear though brief. In performing this function the commentator stands in an appropriate place facing the faithful, but not at the ambo.”
Clearly the priest can say introductory remarks after the Sign of the Cross and the greeting. From the GIRM:
“50. … After the greeting of the people, the priest, the deacon, or a lay minister may very briefly introduce the faithful to the Mass of the day.”
Perhaps the commentators remarks should also be at this time:
“124. … The priest himself or some other minister may also very briefly introduce the faithful to the Mass of the day.”
GIRM n. 31 also specifies this time.