Is there a possibility to create custom window for adding ItemNode in Managment Client?

I mean, like in Xprotect LPR plugin, with custom control buttons. Not only “OK” and “Cancel”

Yes. The way is to make an overwrite for UserControl VideoOS.Platform.Admin.ItemManager.GenerateAddUserControl

Ref. https://doc.developer.milestonesys.com/html/index.html?base=miphelp/class_video_o_s_1_1_platform_1_1_admin_1_1_item_manager.html&tree=tree_search.html?search=itemmanager

A new plugin based on the plugin template will include this, it might be beneficial to explore it on a newly created plugin just by the template..