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Well, it's cutting and pasting, but it's easy cutting and pasting!

Select the list of items to be turned into tasks
Edit->Copy (command-C)
Now click on the row handle of the desired parent (the dot at the left end)
Edit->Paste (command-V)
Edit->Outlining->Indent (command-])

Now you should have a new child action for each line in the selected region with a minimum of fuss.