I came up with a very baroque process to migrate my Workflowy outlines (exported as OPML files) to Outlinely. It uses Hazel, TextWrangler, AppleScript, etc. I honestly don't know how useful it will be to others, but I tried to write it up so that others could copy it:

https://d.pr/dpCP

Sorry, that link seems to have gone bad, try this one:

https://d.pr/2cdx

I shared this with Dat, the developer. He pointed out that you can also import just by control-clicking on a folder in the sidebar. It is hard to discover this feature as it only works with custom folders, not the inbox. My approach has some useful aspects that work better for importing from Workflowy. These would not be necessary if one could simply turn a branch of an outline into a new document, a feature which I have requested and which Dat seemed to find interesting. I don't know however if it will ever be implemented...