Enchant Item Riutal only makes magic items officially stated as items in the game - it doesn't randomly give bonuses to items (by RAW).
You could invent a custom magic item, that the EI ritual would then be able to make. Skill enhancing items generally give an item bonus to a skill at a X+1/X+2/X+3 per tier of the item. The initial level of the item woul ddepend how useful the increase is, the paragon version woul dbe base level +10 (and base level +20 for the epic version).
I say X+1 as the bonus generally as if you made a magic version of somethiong that gave an item bonus by default it would have to give more bonus to be worthwhile.
I would suggest:
Magic Theives' Tools Level 4/14/24
The user of these gains a +4 bonus to Theivery Checks. This does not stack with the bonus from mundane thieves' tools.
Level 14: +5 bonus
Level 24: +6 bonus
Of course I am just spitballing on the level and the exact bonus but that seems viable to me. Such an item may already exist in some book/DagMag but I don't know of it off the top of my head.