Can a helpless non-muslim voluntarily accept slavery to pay off his/debt or to maintain a lifestyle? Where exactly in Quran/Hadith, voluntarily slavery is prohibited?
Yes, Islam prohibits enslavement of a free person, even if he voluntarily accepts it.
Enslaving a free person is categorically prohibited:
قال الله ثلاثة أنا خصمهم يوم القيامة... ورجل باع حرا فأكل ثمنه
Allah says, 'I will be against three persons on the Day of Resurrection: ... One who sells a free person (as a slave) and eats the price
The only exception to this is the case of a Harbi Kafir. For every other case the default prohibition noted above will hold. A free person does not own himself - rather Allah owns him and his freedom is Allah's right which can only be relinquished through His permission.
والحرية حق لله تعالى فلا يقدر أحد على إبطاله إلا بحكم الشرع، فلا يجوز إبطال هذا الحق، ومن ذلك أنه لا يجوز استرقاق الحر ولو رضي بذلك
Freedom is the right of Allah, hence no one has the power to overturn it except with a command from Shariah. Hence it is not permitted to violate this right, and a free person can not be enslaved, even if he consents to it