Combining System and user settings (Concatenating lists)

Hi,

I'm administrating a Linux cluster for several dozens of users. 
What we want to provide are system-wide settings for some configuration options.
For example:
We want to provide a set of company-wide addressbook resources by the following entries:

ResourceKeys=FX93JgNrTp,abcdefgh

On the other hand users have the same entry type in their config files

ResourceKeys=12345678,blblblbl

What I would like to have is something like:

ResourceKeys=FX93JgNrTp,abcdefgh,12345678,blblblbl

Instead only the users config entries are considered, because they overwrite the system-wide setting. The $i flag doesn't help either, because then the user config is neglected.

It there a possibility to achieve concatenate such kinds of entries?

BR
Marc