I've never met answers to your question, but i actually never search hard for it. I don't know how to add custom field to standard reports. But you can create SAP Query reports where you can add those fields.
Choose Logical database for you need.
Create Infoset with tr. SQ02 at first.
Choose all fields that you want to use and then add custom fields.
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Add fields with their description and parameters.
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Create SELECT to select data for these fields
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Add custom fields to the list of used fields for your report.
Then create SQ01 and other things.
