You could create a formula field that you can exclude the weekend days. The problem is that you're saying you cannot add any fields to the report type thus that idea is moot. I think you might be stuck with excel or if you can figure out how to add in fields to the report type, which feels strange that you can't, but if you can then there are examples out there.
Date Formula to Calculate date excluding weekendsDate Formula to Calculate date excluding weekends
Using formula to exclude weekend and holidays in salesforceUsing formula to exclude weekend and holidays in salesforce
Excluding weekend and holidays from my days numberExcluding weekend and holidays from my days number
