All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
It is impermissible to add a prostration for supplication in prayer, whether it is obligatory or voluntary. When one deliberately adds an action to prayer – even if it belongs to it – the prayer is invalid. The supplication during the Sujood (i.e. prostration) position in an obligatory or voluntary prayer is permissible and even required, unless one supplicates with something evil or for severing ties of kinship. It is better to use the supplications reported in the Quran and the Sunnah.
Allaah Knows best.