This is actually quite simple to do.
First look up the item you want to have a tooltip of on wowhead and copy the url of that item's page e.g. http://www.wowhead.com/?item=40419
Then whilst editing your Path of a Hero journal entry, highlight the name of the item and click the insert hyperlink button on the rich text editor user interface (the icon that looks like a chain).
Then paste the wowhead item url into the box that pops up and click OK. Now when you save your journal entry the tooltip should appear when you hover over the item name.
I agree something of this nature would be really awesome and help the site grow by allowing it to spread in game.
The problem is that I have looked into doing this and found that WoW add-ons are completely restricted from accessing external web sites. This means they operate in a sandbox without any outside communication. Therefore it is physically impossible for me to create an add-on that communicates with Path of a Hero in any way :(
At this stage your other characters will not become epic. This is to prevent a situation where people start to pretend to be alts of each other just so that they can share the epicness.
I will rethink this situation once I see how the epic account is going in general.
The epic account is BoP not BoA.