iThoughts can import many of the most popular desktop mind map file formats - see here for a list.
On Windows/Mac it is a simple matter of File -> Open and choose the file to import. On Windows you may need to change the filter in the File Open dialog.
The tricky part (on iOS) is getting the map onto your iOS device in the first place. The following methods will work:
Email the map to yourself then tap on the attachment (in Mail on your iOS device) and choose 'Copy To [iThoughts]' from the Share Sheet.
Copy the map into the cloud (iCloud Drive, Dropbox, OneDrive, GDrive etc.) then close any open map and choose Import from the menu under the Gear toolbar button.
When a map is ‘imported’ to iThoughts then it will be converted to the native iThoughts file format (.itmz) and you will be asked where you’d like it saved. You can then choose the folder and whether it should be stored on device or in iCloud or Dropbox etc.
NB: The imported map is DUPLICATED and CONVERTED to ITMZ format - so any changes will not be written back to the original.
NB2: Not all attributes will be carried over from the source map during the import. For example Freemind has 3 colors per topic (line, fill, text) whereas iThoughts only supports a single topic color. During the import iThoughts will choose one of the Freemind topic colors to use and ignore the others.
iThoughts can export in many different file formats. Click/tap on the Share toolbar button and then choose the method (email, upload etc) and file format to export.
Points to note when exporting:
You can choose to export either the map as a whole or the selected branch.