If the unit will hold the item forever and never drop it, then you can manage those skills just like any other unit skill. The abilities that are inside a spellbook are in the unit, even if it's an item. You can use 'Unit - Set level of ability'
If the item is dropable/acquireable, you'll have to:
Event - A unit Acquires an Item
Condition - Item being manipulated Equal To Terra
- Triggering Unit is a Hero
Actions ->
-> Set level of #1TerraSkill for (Triggering Unit) to *YourFormula*
-> Set level of #2TerraSkill for (Triggering Unit) to *YourFormula*
-> Set level of #3TerraSkill for (Triggering Unit) to *YourFormula*
-> Set level of #4TerraSkill for (Triggering Unit) to *YourFormula*
You'll have to figure out the formula by yourself. If the hero has a maximum level of 50, and the Skills maximum level is 5, The formula would be something like 'Level of (Triggering Unit) / 10'