Both z/OS and LUW have IXF files (the z/OS one is used in QMF), but they are not compatiable.
One way would be to simply pipe a select query to a file, which will produce a fixed format for each column (same starting and ending positions for each column on each row). Then transfer the file to the mainframe as text file (convert ascii to ebcdic).
You may be able to do it easier using the Control Center if both databases are catalogued, but I am not sure about that. I know that you can extract data from the mainframe on the Control Center into an IXF file and then load into DB2 LUW, but I don't about the reverse.