many thanks. It's actually going to be a bit easier to do the move if both libraries are in the same Windows User account. (Although it's still going to be a bit fiddly.)
Open the library you're going to be moving content from. Right-click on items you plan to shift, and select "Show in Windows Explorer", as per the following screenshot:
Make a note of the file locations of the music/content you want to move.
Close that library. Open the library you want to move stuff to. Go "Edit > preferences".
In the Advanced tab, check "Copy files to iTunes media folder when adding to library", as per the following screenshot:
... and click OK.
Now, use the "File > Add ... to Library" commands to add the content from your other library to the new one. (Use the notes you made on file locations earlier as a guide when browsing to the files.)
See the following document for more information about the "Add ... to Library" controls: