Looking at the Scintilla documentation, there’s SCI_MULTIPLESELECTADDNEXT, which says “If the current selection is empty then select word around caret”. Unfortunately, that’s not in the Settings > Shortcut Mapper > Scintilla Commands list. Looking at the Scintilla History, that command was added in Scintilla 3.5.7, but Notepad++ uses Scintilla 3.5.6: if Don had done one more update, that command could have been in the Shortcut Mapper. Sigh.
But @guy038’s macro-suggestion should be workable for you.