Adding to existing registry entries

[gensan]

Can anybody help me on how I can add or append values to existing registry entries? Specifically, my problem is that I am to implement password filtering (using passfilt.dll). I was instructed to add to registry on HKEY_LOCAL_MACHINE, KEY: \SYSTEM\CURRENTCONTROLSET\CONTROL\LSA, Name: NOTIFICATION PACKAGES, TYPE:REG_EXPAND_SZ, VALUE: %SYSTEMROOT%SYSTEM32\PASSFILT.DLL. However, upon going to that location, there is already a value for such. I was only instructed to not overwrite any values. How do I add to the existing values on the registry.

thanks