First idea that popped into my head...
Create an ad hoc task that displays an HTML Form. The form would contain a textarea mapped to a property. The user would paste the values into this fields (the idea being there is one value per line) and submit the form.
Next you need to read those values so they can be added as individual keywords. You could maybe do this with Set Property to Expression. There is a Split function that lets you split a value (such as a property) using a delimiter. I haven't used it myself, so I'm not certain if you could pass a newline character as a delimiter. If you can, you could do this, then set the keyword from the property to add all instances.
If the expression method isn't viable, you could do something similar using a Unity script. You would read the property into a string and then call Split('\n') on it to create an array split by newline. You could then loop through the array, adding each value as a keyword to the document.
Other options could include using a IClientWorkflowScript to display a custom winform to prompt for the values instead of an HTML Form.
There's a chance there's an even easier method I'm not thinking of.
