Strand7 supports import of MSC_NASTRAN Version 70 data files. It expects the file to be called *.dat or *.nas. It is looking for an ASCII plain text file, with strict formatting, 80 characters per line, and the usual three main sections: Executive Control, Case Control, and Bulk Data.
If you just rename the file as either *.dat or *.nas, you should be able to import it into Strand7 OK.
Hope this helps!