Remember five different instructions, one from the 1960’s, one from the 1980’s, one from the 1990’s, and two from the last seven years forbid the alteration of the approved liturgical texts and actions. Sacrosanctum concilium, art. 22. 3; Canon 846.1; CCC 1125; GIRM, no. 24; and Redemptionis Sacramentum, nos. 31, 59 all specifically mandate that no one, not even a priest, may add, omit, or alter the texts of the sacred liturgical books.
The Introduction to the Lectionary for Mass states, “12. In the celebration of Mass the biblical readings with their accompanying scriptural chants may not be omitted, shortened, or, worse still, replaced by nonbiblical readings.”
Redemptionis Sacramentum states, "61. In selecting the biblical readings for proclamation in the celebration of Mass, the norms found in the liturgical books are to be followed, so that indeed ‘a richer table of the word of God will be prepared for the faithful, and the biblical treasures opened up for them’.
- It is also illicit to omit or to substitute the prescribed biblical readings on one’s own initiative, and especially ‘to substitute other, non-biblical texts for the readings and responsorial Psalm, which contain the word of God’."
The GIRM states “357. For Sundays and solemnities, three readings are assigned: that is, from a Prophet, an Apostle, and a Gospel. By these the Christian people are brought to know the continuity of the work of salvation according to the God’s wonderful plan. These readings should be followed strictly. During the Easter Season, according to the tradition of the Church, instead of the reading from the Old Testament, the reading is taken from the Acts of the Apostles.”
and again, “57.Moreover, it is unlawful to substitute other, non-biblical texts for the readings and responsorial Psalm, which contain the word of God.”
If the morning choir wishes to participate, they could lead a service of carols before Mass, and then sing the evening choir’s programmed music. Readings could be added during the carol service.
I’m not sure if it’s allowed to join Evening Prayer 1 of Christmas with the Vigil Mass. If so, Mass could be extended that way. Other than combining an hour with Mass, I don’t believe there is any addition of lessons allowed.