Can radiation therapy for lung cancer also treat a pneumocystis infection?

No. Pneumocystis is an infectious organism. It can only be treated with specific antibiotics. On the other hand, radiation can weaken the immune system and cause pneumocystis pneumonia.
No. Antibiotics treats pneumocystis infection.
Unlikely. Pneumocytis infections usually require high-doses of antibiotics such as Bactrim (sulfamethoxazole and trimethoprim) (sometimes steroids too) -radiation is unlikely to treat pneumocystis or any infection.