Open tooltip.2da with ifinity explorer and export it.
Then open that in a text editor. Go to the bottom and add your item's file name sans .itm .
Then load IEEP. Go to dialog.tlk editor. Pick the write option and write what you want the tooltip to say. It will tell you the string reference (strref)of the tooltip.
Go back to tooltip.2da . The 1, and 2 at the top are the extension headers of the items. Paste the strref of the tooltip for the header in the header's column( ex. if the ability for the item sw1h04 in the first extension header should have a tooltip called thrown and thrown has a strref of 23456, put 23456 under the 1 next to sw1h04).
You can add numbers past two at the top if you have an item with more than two extension headers. If you want the tooltip of a header to just be the name of an item put a -1 in the column.
Remember, the columns have to be lined up right so turn off word wrap, margins etc. in your word proccesor. After you have done this save the file as tooltip.2da in the overried folder.