It's easy. Check the Reagent Pouch xml structure and do the same ;)
Item xml
<set name="handler" val="ItemSkills" />
<set name="item_skill" val="2174-1" />
==>
skill xml
<skill id="2174" levels="3" name="Ingredient Bag">
<table name="#extractableItems">
6019,2,30;6013,2,30;6014,1,20;6016,1,20 <--- here, it's level 1 skill
6017,2,10;6020,2,20;6014,1,35;6016,1,35
6012,1,20;6018,2,20;6019,2,20;6013,1,40
</table>
<set name="capsuled_items_skill" val="#extractableItems" />
<set name="operateType" val="OP_ACTIVE" />
<set name="skillType" val="EXTRACTABLE" />
<set name="target" val="TARGET_SELF" />
</skill>