It is not permissible to use prayers not found in the Sacramentary as the Opening Prayer, the Prayer over the Gifts, the Preface, the Eucharistic Prayer and the Post-Communion Prayer. Gone are the days when presiders pray according to their ability - it hasn't been allowed for the last 14 to 15 centuries. It was during the time of the likes of Tertullian, St. Ambrose, St. Leo the Great, St. Gregory the Great, that the prayers for the Mass started becoming formula prayers. The reason why they started prescribing and regulating these prayers is due to heresy. Some priests made prayers whose theologies are dubious. The rule now is for priests to celebrate the liturgy as handed down by the Church - this is part of their priestly vows.
I am now taking up Liturgical Books and one thing I am able to appreciate is that our prayers today date back to the time of the people I mentioned above. It's comforting to know that prayers said at mass are made by saints. For example, the Opening Prayer at the Vigil Mass of Christmas was written by St. Leo the Great.
Funny how some people feel they are better than the likes of St. Leo the Great.