Add features to an application developed with symfony (Billing tool)

The tool is already existing. Actually the application permitted : Create invoice Create estimate Create an invoice from an estimate Change invoice's state Validated payment Share the invoice with customer Recurring Expenses We want to add different features to this tool: Online Invoicing Customize your Invoice Online Payments Expense Tracking Automatic Expense Import Attach PDF or Image Receipts Assign & Rebill Expenses Time Tracking Timer Team Timesheets Track Unbilled Time Different Rates for each Project Accounting Reports & Taxes Profit and Loss Accounts Aging Expense Reports Sales Tax Balance Sheets