I don't think they would be, mainly because DLL skins are installed in a different way to Plus! skins and it will confuse people with both.
If you have modified the DLL and use the Messenger Plus! extraction process then it will extract your modified resources. It wouldn't be too hard to convert it to a .plsk.
All you would have to do would be to write a SkinInfo file and package it. You may have to modify some files here and there depending on what Messenger version you build it for. For information on SkinInfo check out the tutorials section (note they are written for Messenger 8.5, but the concepts are the same for 9)
http://www.msgpluslive.net/skins/resources/
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