We May Superadd Prayers of Our Own to the Lord's Prayer
Since, however, the Lord, the Foreseer of human necessities, said separately, after delivering His Rule of Prayer, “Ask, and you shall receive;” and since there are petitions which are made according to the circumstances of each individual; our additional wants have the right— after beginning with the legitimate and customary prayers as a foundation, as it were— of rearing an outer superstructure of petitions, yet with remembrance of the Master's precepts.
Source: On Prayer (New Advent)