You can add additional keyboard layouts frorm the KDE control panel (find this in the K menu, or press alt+f2 and enter "kcontrol".
In the search dialog, enter "keyboard" and then click "keyboard layout" from the list below.
Select "Enable Keyboard Layouts" at the top.
In the the list "Active layouts", select your keyboard layout, and below under "layout variant", select intl (international), and make sure "Include latin layout" is enabled.
You should now be able to enter accented characters by pressing an quote type and then the letter to add it to. Thus
"a becomes ä
~i becomes ĩ
`e becomes è
You can also set up multiple layouts and switch between them from an applet or by binding a global keyboard short cut to it. I do this to switch between UK normal, and UK with international characters.