Read and writing new line to multiline registry string

I've been a bit confused with some of the registry examples I've seen so far...

Theres a registry string that may or may not exist. If it doesn't I want to add it. I need to check this string and if it does exist I need to write a new line without overwriting the previous data.

How can I achieve this?

Sorry if the question is vague.

Thanks

Dim WshShell As Object
Dim sSection As String

Set WshShell = CreateObject("WScript.Shell")
sSection = "HKEY_LOCAL_MACHINE\Software\YourAppName\"
'Check wheather your string exist
if WshShell.RegRead(sSection & "DBName")="" then
'Write to register
WshShell.RegWrite sSection & "\" & "DBName, "YourString"
endif

Set WshShell = Nothing