Is a "lam" added to the end of the word "الصراط" (As-Sirat) to become "الصراطل" (As-Siratol), and what is the ruling on reciting it as such in prayer?

If you wish to pronounce "al-Ṣirāṭ" in Surat al-Fatiha, the lām after the ṭāʾ is pronounced directly when connected to "al-Mustaqīm," because the hamza of "al-Mustaqīm" is a hamzat waṣl (connecting hamza). However, if you pause at "al-Ṣirāṭ," the ṭāʾ is given a sukūn and the lām is not pronounced.

One who makes a mistake in reciting this during prayer should be advised. Deliberately making a mistake invalidates the prayer and invalidates leading the prayer with him, unless the meaning is altered in a surah other than al-Fatiha, in which case the prayer is not invalidated unless it was done intentionally.

It is recommended to learn tajwīd and correct recitation from a proficient shaykh.