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For the 6 fields listed above, this is what my complete list of call number fields would look like, right before I export them from Create Lists:
This is what my file looks like when I open it in Notepad. Note that you should see semicolons (;) in between the call number subfields. This is what Ex Libris uses to migrate the call number fields to the separate $h and $i fields in the holdings record.
Note that in my file there are now column headings that are identical (I have two 050|ab headers, two 090|ab headers and two 086|abcd headers). Because of this, I must alter my items file so that these duplicate headers don't exist in the file. If you are only exporting call numbers from either the bib record or the item record, you can skip this step. This step only applies if you are exporting the same fields from both the item record and the bib record.
The column headers can be manually modified, but you can't do this modification in Excel, as doing so can impact the integrity of your data. Open your item file in Notepad or a similar text editor in order to change the headers in your items file so that every column is unique. This is what my modified items file looks like now. I only need to change the last three columns in order to make every column header in my file unique. 
The column headers that you modify can be whatever you like. However, they must match whatever you place onto your Field Mapping form under the Items tab call number section.
Note that anything that is in the CALL #(ITEM) field will override all other call number fields. The CALL #(ITEM) field and the CALL #(BIBLIO) fields never retain the $h and $i upon export from Millennium and Sierra. If it is important to retain these subfields, you can choose not to export CALL #(ITEM) or CALL #(BIBLIO) at all.
If you do wish to include the CALL#(ITEM) field, you may do so, but turn off the setting under Settings | Create Lists | "Look up call numbers". Leaving this setting on will cause the CALL #(ITEM) field to be fully populated, and will subsequently override all other call number fields. If it is important to retain the $i in the holdings call number, I would suggest not exporting the CALL #(ITEM) field from Sierra/Millennium at all.
This is what my Field Mapping form looks like for the situation outlined above. Note that I am not including the CALL #(ITEM) field at all. My modified headings '090|abitem', '050|abitem' and '086|abcditem' are listed before the bib record call number MARC fields, because I want the item call number fields to take precedence. If you wish for the bib record call number fields to take precedence, you would list the corresponding MARC fields for the bib record call numbers first.
