Why was the rice component added to 气 to form 氣?
說文解字 has both references for 氣 and 气.

Put simply, 气 and 氣 comes from different origin and somehow merge together into one stream later.

In detail: 米 is not added to 气 to get 氣 in Tang Dynasty. Long time ago, 氣 appears in the form of Bronze inscription, though different meaning as 气. 氣 is originally designed to mean the "air generated from digesting food". While 气 is originally designed to mean the physical gas. In the reference of 氣, "饋客芻米也" means "氣 is a verb, showing the action of giving guest fodder (芻, chu2) and rice (米) as gift." 饋 (kui4) is "to give someone gift". 氣 with this meaning later becomes 餼 to clarify the action relating to food.

So nowadays, in Taiwan, 氣為餼之異體 (氣 is another non-standard variant of 餼). Also, according the 教育部異體字字典 气為氣之異體 (气 is another non-standard variant of 氣). In China mainland, 氣 is replaced by 气 as standard.