There's the ":a" command, but that's multi-line, and argdo asks you for the text again for each file.
The docs mention the global command (g/pat/command) that will use an alternative version of ":a" that is terminated by a newline instead of by "." on a line (you can include newlines by escaping them with "\"). But I couldn't get this to work.
The only way I've seen is to first yank the text-to-be-added into a named register, then use:
:argdo put x " where x is the register
I'm hoping for something like
:argdo append myTextHere
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If you want it after the current line:.
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Inserting text with linebreaks in it:.
:argdo exe 'normal osometext'
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To append text to the line 10:.
:argdo call append(10, "hello world")
:argdo call setline(10, getline(10)."textappended")
You can paste that buffer in insert mode for example by typing
So all you need is
argdo put ".