I'm going to assume that you mean you want to go back to the LotRO UI
skin as it was before the F2P switch. (Using the word "plugin" implies you want to do something with LUA scripting.) Of course the answer isn't a simple yes or no. So here goes...
Yes, it is technically possible to revert parts of the UI to the way it was before Vol 3 Book 2 (F2P). You would start with the Book 9 skin art pack in
LotRO Tools & Utilities for Developers as compared to the V3B2 skin art pack. However, Updates 1 and 2 changed several parts of the UI. We don't have an art pack for those changes so nothing from those updates can be re-skinned (unless you're really good at guessing/haxoring). Probably most of the changes you want to undo are from Updates 1 and 2.
Feel free to beg on the
Turbine forums.
As far as I know, no one has made a skin like you want. Personally I wouldn't even try without the Update 2 skin art pack but hopefully someone can help you. Good luck!