That option is in the Advanced section of the Row Inspector, and only appears in OmniOutliner Professional (I don't recall which version you have).
If you open your RTF file in something like TextEdit, select the whole thing, Edit->Copy, and then Edit->Paste into OmniOutliner (having first clicked on a row handle where you want the material pasted) you should get a row for every paragraph that you had on the clipboard. Might have to go through and delete some blank lines, depending on how the original was formatted.