Go to Applications -> Modules -> (edit) Custom module -> User interface.
Find E-commerce reports UI elements in the tree. (I suppose you want to add another order report)
Create a new UI element under CMS -> Administration -> E-commerce -> Store reports -> Orders.
Set element Type to URL.
Set element Target URL to ~/CMSModules/Reporting/Tools/Ecommerce/Report_View.aspx?reportcodename=YOUR_REPORT_CODE_NAME&displayfilter=1
The new report should be visible at Applications -> E-commerce -> Store reports -> Orders.