Is it permissible to omit the Salawat al-Ibrahimiyyah in supererogatory prayers?

The voluntary (supererogatory) prayer is like the obligatory prayer in its conditions and pillars, except for what is specifically exempted. As for sending blessings upon the Prophet (peace be upon him), scholars have differed regarding whether it is a pillar (integral part) of the prayer or merely recommended:

The Shafi'is and Hanbalis consider it a pillar. The Hanafis and Malikis consider it recommended.

Most of those who hold the view of its being a pillar believe that sufficiency is achieved by saying: "Allahumma salli 'ala Muhammad" (O Allah, send blessings upon Muhammad), and that completing the Ibrahimiyyah prayer is recommended. What is obligatory in the voluntary prayer is for the worshipper to say, after the tashahhud which is followed by the salām: "Allahumma salli 'ala Muhammad." However, it is more preferable to complete the Ibrahimiyyah prayer to conform to the Sunnah and to avoid scholarly disagreement. There is no difference in this regard between the voluntary and the obligatory prayer.