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.
Not signing Riba-related documents does not keep you away from bearing sin and helping doing it as long as you prepare them. It was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, cursed the one who consumes Riba and the one who pays it, the one who writes it down and the two who witness it, and he said: They are all the same." [Muslim and others]
Accordingly, the curse included the one who writes it down for he helps doing it by writing it. The same ruling applies to all who help fulfilling it. So, fear Allaah The Almighty and avoid, in your work, all what is related to Riba-based transactions, even if this requires you to leave that job and to search for another one that involves no prohibitions nor helping doing them. Whoever gives up a thing for the sake of Allaah, Allaah will compensate him with something better.
As for the question of answering supplication, then amongst the things that prevent answering it is the prohibited earning. Also, amongst the causes of accepting and answering it is to consume lawful things for the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: "O Sa‘d, earn your livelihood lawfully and your supplications will be heard." [At-Tabaraani in Al-Awsat]
Allaah Knows best.