Gain extra damage and damage conversion

Does the mod Gain X% of Damage as Extra Damage apply before or after conversions?

If I have all of Glacial Cascades physical damage converted to Cold, Lightning, and Fire, does Gain X% of Physical Damage as Extra Cold Damage not actually add any?

It will work on your base physical damage just fine.

It works, because converted damage retains the old type as well when it comes to modifiers.