In the phrase, "j'arrive à manger du riz", how does one rephrase it using the indirect object pronoun method. Since lui, leur refer to people.

J'arrive à manger du riz. -- I manage to eat rice.

Which noun do you want to replace with a pronoun? There is only "du riz". You can't replace manger du riz as a group with a pronoun since it is an action.

J'arrive à en manger. -- I manage to eat some.