@LBouchard,
The Excel Add-In uploads data specific to the fields of the selected entity (i.e. - The TEST fields) and does not cross entities.
The Test cases are linked to Requirements via the REQ_COVER database table. Therefore, there is not a specific Test field that determines Requirement linkage.
Unfortunately, this means you would not be able to add a specific column to your wotksheet and have the Excel Add-in utilize it for Requirement linkage.
An alternative approach may be to use the ALM/QC API inside of an Excel worksheet (think VBA and macros) to mimic the functionality of the Excel add-in while building additional functionality for Requirement linkage.