If you're using KDE, you can easily do it using the extremely flexible Shortcuts module (one of the many reasons I love KDE). Try this:
- In Configure Desktop, type Custom Shorcuts into the search box and open the respective dialog (that is Common Appearance and Behavior > Shortcuts and Gestures > Custom Shortcuts, in the old tree-style mode)
- Select Edit > New > Global Shorcut > Send Keyboard Input
- Now, on the Trigger tab, enter your desired shortcut key combination (I tried it with Alt+N)
- On the Action tab, type 'Down' into the box (without the quotes)
- Click Apply
Alt+N should now be equal to pressing the down key. (It works analogously with the keyword 'Up')
The Shortcuts module allows you to limit your configuration to Eclipse (or any other program) too, should you desire so.