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Probably the easiest thing to do is to fire up OmniOutliner, paste the rows in there, being sure to click on a row handle before pasting. This will get you a document with one OmniOutliner row per line on the clipboard. Now you can either copy and paste into the outline view in OmniGraffle, or save and open the OmniOutliner file in OmniGraffle.

For some reason, OmniGraffle doesn't seem to follow the convention used by other Omni apps that pasting after selecting an entire row gets you multiple rows instead of a single multi-line row.