Migrating is for moving content from one project to another. Given you’ve also asked this under this question, I assume you’re dealing with content for the same project, but are moving it around without version control.
You should be able to just place the .uasset files in the appropriate location in the Content folder for your project via explorer/finder; these should then appear in the content browser correctly. This is what happens when the .uasset files are placed under source control.