Q: Is it permissible to make Dua in sajdah after performing Farz, sunnats and nafl?
A:It is permissible with these three conditions:
1) You are in seclusion and others cannot see you because they would
Understand it as part of salaah.
2) You don’t make it a habit.
3) You don’t understand it as sunnah or mustahab. (Ahsanul Fataawa)