Hello Lisbeth,
I am sorry but you are taking a completely wrong approach to the problem. Opening an XML file in FM and saving it as an FM file does not make it unstructured and it never will, or should. I understand your problem as follows: you have the XML documents and need to apply the formats from an unstructured FM template to the various elements.
There are two different options I can see for you:
1. Change the EDD for the structured application that is used to import the S100D documents into FM. This is not the easiest job in the world but it can certainly be done. Instead of assigning the formats that happen to be in the template that FrameMaker uses for importing S1000D documents, the EDD would then assign the formats from your unstructured document to the elements. There are various developers on this forum - including myself - who would gladly do this work for a reasonable fee.
2. Open the template that FrameMaker uses to import S1000D documents and adapt the styles in that document to the ones you have in the unstructured template. In this case, the names of the formats in all the paragraphs of the imported document will not change to the names used in your unstructured template but the end result will look the same. This work is tedious and may take quite some time, but it does not require a lot of in-depth knowledge of structured applications and EDDs.
If the formats have to have the names of the styles defined in the unstructured template, option 1 is the only one for you.
Kind regards
Jang