To add a runtime folder, it needs the correct structure, or it will appear empty.
Example:

I have to add the Antonia folder to add a new Runtime, under this name,
That being said, even if you add something from one runtime, Poser will always automatically scan each runtime registered before the one in use to load the resources it needs (it can be an external geometry, the bitmaps, or scripts that are referenced in  the Poser file.
Example with Erogenesis' Combo character: you load a character, and a choice appear because of its internal content:

{
version
{
number 10.04
build 28925
runPythonScript ":Runtime:Libraries:Character:Evolution:Figures:Characters:load_current_version.py"
runPythonScript ":Runtime:Libraries:Character:Evolution:Figures:Combo Characters:load_Character - Kaylee.py"
}
}

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