I'm wondering if it is possible to add (install) LUCI plugin (i mean add new element or tab to the current webUI) without the need to compile the entire image, just like creating and installing new package?
Sure it is. You'll need to write some code in Lua though, for anythink that is not completely static. The Lua code gets interpreted, no compilation involved.
BTW. please do not shout! (thread title)