Weeping during prayer is one of the outcomes of humble submission (khushu‘) in prayer. As for making oneself weep, there is a da‘if (weak) hadith that has been narrated concerning it; if it is sahih, what is meant is bringing sorrow into the heart and focusing, not making sounds by weeping artificially.

If someone is unable to sit in the Sunnah manner between the two prostrations or in the tashahhud, it is permissible for him to sit resting on his knees.