@Gouki sorry for not being specific enough on my last comment
but i actually mean something like the following :
<passive_effect name="ModSlots" operation="base_add" level="1" value="1,1,0,0,-1,-2" tier="1,2,3,4,5,6"/>
<passive_effect name="ModSlots" operation="base_add" level="2" value="2,2,1,1,0,-1" tier="1,2,3,4,5,6"/>
<passive_effect name="ModSlots" operation="base_add" level="3" value="3,3,2,2,1,0" tier="1,2,3,4,5,6"/>
<passive_effect name="ModSlots" operation="base_add" level="4" value="4,4,3,3,2,1" tier="1,2,3,4,5,6"/>
<passive_effect name="ModSlots" operation="base_add" level="5" value="5,5,4,4,3,2" tier="1,2,3,4,5,6"/>
<effect_description level="1" desc_base="2 Mod Slots" long_desc_key="Now you will have 2 mod slots on your items, gear and weapons."/>
<effect_description level="2" desc_base="3 Mod Slots" long_desc_key="Now you will have 3 mod slots on your items, gear and weapons."/>
<effect_description level="3" desc_base="4 Mod Slots" long_desc_key="Now you will have 4 mod slots on your items, gear and weapons."/>
<effect_description level="4" desc_base="5 Mod Slots" long_desc_key="Now you will have 5 mod slots on your items, gear and weapons."/>
<effect_description level="5" desc_base="6 Mod Slots" long_desc_key="Now you will have 6 mod slots on your items, gear and weapons."/>
except something that does work