Azunai , will you help me? please write or paste the steps on how to do that.
i don't have much experience with modding, so i just tried the most obvious solution without cheching tutorials or anything. seems to work.
i don't know how to make a proper mod, so i basically just "hacked" the game XML file that contains the relevant information. i think save games store the XML data, which means it will only work on a new game.
step 1: find the relevant file; on my machine (win7 64bit) that's the file
"CoreAbilities.XML" in "C:\Program Files (x86)\Steam\steamapps\common\FE Legendary Heroes\data\English"
step 2: make a backup copy of the file; if something goes wrong, you can delete your failed attempt and replace it with the backup.
step 3: open that file with a text editor. i recommend the freeware tool "Notepad++", that's a basic text editor with options to XML tags and similar stuff; any other text processor will work, though - the default windwos notepad will get the job done
step 4: technically, you would now look for the Plate ability and essentially copy & paste the ability of the warrior or defender and rewrite some of the tags so it works for mage. you don't have to do that, since i already did. just copy the following section:
<AbilityBonus InternalName="ArmorProficiency2Ability_Mage">
<AbilityBonusType>Unit_Level</AbilityBonusType>
<AbilityBonusOption InternalName="ArmorProficiency2Ability_Mage">
<DisplayName>Plate Mail Proficiency</DisplayName>
<Description>Allows the unit to wear Plate Mail armor</Description>
<Icon>Ability_PlateMailProficiency_Icon.png</Icon>
<PerkTreePosX>0</PerkTreePosX>
<PerkTreePosY>2</PerkTreePosY>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_ArmorProficiency</StrVal>
<Value>1</Value>
<Provides>Allows the unit to wear Plate Mail armor</Provides>
</GameModifier>
<Type>Defensive</Type>
<UpgradePathID>ArmorProficiency</UpgradePathID>
<Prereq>
<Type>AbilityBonusOption</Type>
<Target>Unit</Target>
<Attribute>PathOfTheMage</Attribute>
</Prereq>
<AIData AIPersonality="AI_General">
<AIPriority>0</AIPriority>
</AIData>
</AbilityBonusOption>
</AbilityBonus>
step 5: insert the new ability in the file. that's the tricky part. it doesn't really matter where exactly you paste it, but you must make sure that it's between the end of one ability and the beginning of the next. if you mess up that structure, the file probably won't load and the game may hang up or crash or whatever.
find the end of an ability, indicated by
</AbilityBonus>
and insert the new ability between that tag and the beginning of the next ability, indicated by
<AbilityBonus InternalName="blabla">
step 6: save the file
step 7: start LH, start a new game; i recommend you make a custom hero and give him the "veteran" trait so you get your first level up instantly to verify whether it worked. pick path of the mage and you should see plate armor proficiency as a new trait, available right at the start (you could probably play around with that stuff to give it another prerequisite. if you think it's too good for level 3, just be disciplined and pick it later 
)
let me know if it worked
btw: if one of the experienced modders gives you a better description, just follow them; as i said, i'm not experienced with that stuff. in fact, that's my first "mod" 
