Is it permissible to practice tawarruk in nafl prayers, and what is the ruling on doing so in them?

It is sunnah for the worshipper to sit mutawarrikan (tavarruk) in the final tashahhud in prayers that have two tashahhuds (i.e., three-unit and four-unit prayers), and to sit muftarishan (iftirash) in the first tashahhud and in two-unit prayers. Scholars have not differentiated in this regard between obligatory and supererogatory prayers, and the matter is broad. Although the Sunnah is tavarruk in the final part of the prayer and iftirash in other parts, a specific posture is not mandatory for the prayer to be valid.