Is there a way to modify an existing value in HKLM\SYSTEM\CurrentControlSet\Services\Eventlog\System without manual editing?

The value (Sources) looks like this...



and I need to insert something in the long list. I've tried reg add but that asks me to overwrite. I presume importing a reg file will do the same. Setup programs can do it so I'm hoping there's a way.

Any ideas?

There is, but you're gonna have to learn a dev language if you don't know one already, and write some code. The reason programs can do this is because the developer has taken a value, read it in as a string, created a new value, and written it to the registry via the RegSetValueEx function in the application's code.

? along the same line, a BAT/CMD script to dump, append, and reload ?