Yes the expense receipts can be added to an expense sheet without opening the expense sheet.
* Click on the Expense icon in the top menu.
* In the expense sheets for 'user name', you can see the list of open expense sheets.
* To edit the receipts of the any of the listed expense sheets, click on "Receipt List" under 'Expense Receipts' section.
* In the 'Expense receipts' pop up box you can click on the "Upload image" under the 'Upload' section.
* Browse and add the expense receipt.