Yes, your customized Web20 skins will work if you register them with RadStyleSheetManager.
However, in order to prevent RadControls from registering automatically their native embedded Web20 skin, you should set EnableEmbeddedSkins="false"
On a side note, using custom skins with the same name as native embedded skins is not very recommended, as it might lead to CSS collisions and unexpected visual appearance (for example if you use a customized and a native Web20 skin on the same page). This might happen in specific scenarios only, but you should keep in ti mind in case something "strange" happens.
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