I have read that when using malted barley the conversion rate of starch to sugar is inefficient.
Good malted barley can self-convert itself. This means that, given enough time and proper temperature, all starch will get converted to maltose, glucose and other fermentables. There is enough enzymes for that.
Of course, when mashing for industry, time is at premium. It may be cheaper overall to boil barley, even malted, then cool it down to mash temperature, and add artificial enzymes. That way starch is "gelled" and easily accessible to enzymes, and you can cut time from 2 hours to possibly as short as half an hour. If you mash only small batch for yourself, it is not an advantage.
If you want to add enzymes, you need amylase. Preferably Beta-amylase, it cuts starch to shorter, easier to ferment sugars. I always try to keep a sachet or two of it, in case I overheat my mash tun. But most of the time, malted barley provides all enzymes I need.
Amylase are the enzyme needed for saccarification (conversion of starches to sugar) beta-amylase / alpha-amylase
As for the malted barley, It depends on the source and how it's malted if it will have any diastatic power (enzymes) or not. Brewers Malt will. Feed barley may not. Flaked, torrified and rolled will not.
You can add Amylase to any grist using 6-row brewers malt or Amylase powder. You need an average of 30°L Diastatic power for complete starch conversion.
6-Row has 160°L so 20% of milled 6-row in any grist will make it convert well.
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