Is is possible to add a class range as loan options? For example class D and lower?

Do you mean allowing only teams of a given class range to loan your players?

Yes.

Is it currently only possible to loan players to B class clubs? They were the only ones interested in my player so far (that I have put on the loan list something like eight times).

Not at all. Each different club class (when CPU controlled) will prioritise loans, first team players and future players in different orders. B clubs if I’m not mistaken would prioritise future players, then loans, then first team players. E and F should be prioritising loans.