Hi, take a look at the usage samples:
https://github.com/fo-dicom/fo-dicom/#file-operations
1. to create a new DicomFile you first have to create a new DicomDataset. Then you add all the dicom tags to this DicomDataset. When you are done, then write
(new DicomFile(dcmDataset)).Save("output.dcm");
2. of course this is possible. After you have created the template dicom file you can open it, change the Dataset and save again. thats prety exactly the sample shown in the link above. But do not forget to change all UIDs (SopInstanceUid, SeriesInstanceUid, AccessionNumber, ...).