you cant do it, combed every inch of the text commands and you simply cant.

Reasons why you would want to.

Add a healing potion to a self healing macro
Add a Vitality potion to the beginning of a self healing macro
add a resistance potion to the end of defensive macro for bosses who use elemental attacks
add a poison/silence/paralyze potion to end of offensive macro
add a specific debuff potion to a defensive macro for boss specific fights that apply poison/silence/paralyze/petrify
add a buff potion to offensive macro such as +intelligence then a group of offensive spells for BLM, as an example