In iTunes for PC, I wish to add two extra info columns to my library. How do I add these as data input lines/spaces/info fields in the "Get Info" box?

I'm not sure what you mean by "show", since that term is not on the menu:

... but I'm sure there is a solution. Perhaps you mean Movement Name which, for reasons best known to Apple, appears on the Get info tab as name. You will see it in one of the screenshots in a moment. If not, then possibly, it will be grouping (more on that also, in a moment).

As for the Work, Use work and movement is already on the Get Info tab. You simply have to turn it on:

Here's an album in my collection that already had the work and movement information filled in. (I don't use it myself, so I'm not sure whether the information is correct, but that's not the point):

Things to note in my screenshot:

• in the box named work, the term mixed appears, because the selected tracks do not have the same information (in other words, the information is mixed)
• the dash in the name field is there because that field also has mixed information (so it might have made more sense to have the term mixed in there as well, but that's not in my control)

If name does not suit your purpose, perhaps you could use grouping, which does appear on the menu. You can use multiple terms in the grouping field and then create Smart Playlists that include tracks that have your grouping word in them.

Do you mean in this "Get Info", seen when selecting several songs and click on Song/Get Info?

To my knowledge, you can't.

However, if you mean this:

Right-click anywhere on that bar to open the full list, select the item and that's it.

I don't recall any time when you could add fields to the Get Info. Perhaps the field you want is on another tab, for example the Options tab. Which field are you trying to edit?