This isn't working for me.  I have the Token dialog open in Quick Fields and have three lines.  When the session runs it combines the three lines into one.  If I actually type line break, then the text of "line break" appears in the metadata.  Thoughts?

This is not currently possible.

Edit: To expand on it, if the data being assigned to the field has newline characters in it, Workflow will not strip them out. But when typing in values in single line fields, Workflow will ignore newline characters. You can still get multiple-line values from newline-delimited multi-value token. You just can't type them in directly.