Holding left Alt key while selecting enables the quick column editing mode. The quick column editing mode is like column editing mode enabled with Column - Column Editing (Alt+C) with the difference that column editing is enabled only for exactly 1 modification. After the modification was done (replace rectangular selection with contents of clipboard, insert a text in selected columns, overwrite text in selected columns by typing, ...) the normal editing mode is re-enabled automatically. If you need column editing for more than 1 action, you have to enable column editing with Alt+C and when finished with modification in column editing, press again Alt+C to turn it off. See the symbol for column editing command in the toolbar to see at any time if column editing is currently active or not.
UltraEdit v18.00.0.1034 has as additional command Paste Column in menu Edit, but by default only in standard environment (using Advanced.mb1). The command is not available in the other preconfigured environments (System Administrator, Web Developer, ...). I don't know why. This command pastes the clipboard contents in column mode even if column editing mode is not explicitly enabled.
Paste Column is available in the menu and the toolbar customization dialogs for adding this command to any customized menu or customized toolbar.
There is by default no hotkey assigned to command Edit - Paste Column. But command EditPasteColumn is available in the Key Mapping configuration dialog and therefore it is possible to assign any hotkey to this command for quick execution.
The Paste Column command was introduced with UE v18.00.0.1029. Previous versions of UltraEdit do not have this command.