Is it permissible to convert a debt amount into a share in the debtor's business, so that the creditor receives a monthly profit from it? And is this considered profit from what has not been guaranteed?

It is not permissible to use a debt as capital in a company, whether it be a mudarabah (profit-sharing) partnership or any other type. This is because a company requires its capital to be a present asset, not a debt, and because there is a suspicion of usury in such an arrangement. A debt is an absent fund held in liability, which contradicts the purpose of a company, which is to realize profit from trade. Furthermore, there is a suspicion that the creditor might delay collecting his debt in order for the debtor to increase it through the pretext of a mudarabah.

In summary: It is not permissible to convert a debt into a share in a company unless the creditor collects his debt first and then invests it in the company.