I've read something about creating a folder link in the eclipse install dir
and in this folder create files with extension lnk. The link-file then
contains the directory where additional plugins and features are located.
The problem is I don't know the URL anymore.

Daniel

You can add extra extension locations to Eclipse.
The important thing is that a directory serving as extra extension
location must contain an (empty) marker file with the name
'.eclipseextension' (without quotes).
To make unpacking of plugin jars easy, I'd recommended to give the
directory containing '.eclipseextension' the name 'eclipse'.

mkdir ~/myextloc
mkdir ~/myextloc/eclipse
touch ~/myextloc/eclipse/.eclipseextension

Unpack any your plugin jars in to ~/myextloc. Most plugin jars have
'eclipse/' as the root path of their entries, so unpacking them
automagically creates the proper structure, i.e. with subdirs:
   ~/myextloc/eclipse/features/
   ~/myextloc/eclipse/plugins/

Then, in Eclipse use Help -> Software Updates -> Manage Configuration.
In the configuration manager, click the 'Add Extension Location' link in
the right pane (or right-click on the 'Eclipse Platform' entry in the
left pane and select New -> Extension Location). Browse to
~/myextloc/eclipse and select it.