When you export, the exported file is a copy of the file in DT. Any editing changes made to one (the original in DT or the copy at the Finder) are no longer synchronized with the other (the copy at the Finder or the original in DT)
When you index the file, any changes you make in either working environment, Finder or DT, will be syncronized.
Importing and indexing the same file is a way of trying to have your cake and eat it too. Except that DT will allow you to do this. And then, at some later point in time, you may have to decide whether you just ate the original and kept the copy, or you ate the original and kept the copy. Or both. Or neither.
Yes. Decide first why you want to do this and whether you really must do this. See also the discussions at this thread. And search for phrases such as convert import to indexed.
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JJW