Funny you ask this since I tried doing the same thing.
So Freeplane has the option to export mindmaps to the OPML extension, which is a standard format for outlines.
And Logseq in theory has the option to import from OPML, however unfortunately currently it does not really work:
I have tried to import OPML files to Logseq with no success, and I would love it if it worked because I have a ton of mind maps. Let’s hope they fix this feature.
Another option from Freeplane is to export to markdown, which will generate a text file with lines whose indentation levels are equivalent to the number of #. That does not work in Logseq but using something like VSCode you could replace the # with tabs and add a - before each text. You also need to delete empty lines. Not ideal, but could work. Maybe I will write a script for that.