Is extending the alif in the word "Ya Rabb" forbidden?

There is no harm in supplicating with "Ya Rabb" (O Lord); it is prescribed in the Qur’an and Sunnah (definition). As for extending the "Ya" in the supplication, it is extended with a natural elongation (madd tabie'i) of two vowel counts. Any increase beyond that is a mispronunciation (lahn) in the recitation of the Qur’an and Hadith (definition). However, outside of those contexts, an increase is not forbidden, though avoiding mispronunciation is better. Allah accepts supplication from a sincere heart, whether it is recited with proper Arabic pronunciation or with mispronunciation.