Using the Notepad++ Python Script plugin, you can use Python to perform complex editor operations with a single button. This script replaces the '<' and '>' characters with the HTML equivalents to enable copy-pasting source code into HTML.

Convert C++ to HTML.py

# First we'll start an undo action, then Ctrl-Z will undo the actions of the whole script
editor.beginUndoAction()

selectionLength = editor.getSelectionEnd() - editor.getSelectionStart()

start = editor.getSelectionStart() if selectionLength > 0 else 0
end = editor.getSelectionEnd() if selectionLength > 0 else editor.getLength()

# editor.replace("&", "&", 0, start, end)
editor.replace("<", "&lt;", 0, start, end)
editor.replace(">", "&gt;", 0, start, end)

# End the undo action, so Ctrl-Z will undo the above  actions
editor.endUndoAction()