Customize Orangehrm

We need to do some customization's to the open source orangehrm. Here is the link, let me know your thoughts on this. [login to view URL] One of the primary requirement is the timesheets section. In the existing version, the employees are able to enter weekly timesheets. They are also able to edit the past timesheets. Our requirements is a) Employee should be able to enter the timesheets in weekly, bi weekly or a monthly format. Most of our employees work on the client location. So they will enter the hours worked and also they will upload a client approved timesheet. b) For each format, (For example weekly timesheet format, the employee is able to enter the hours for each day. Along with this the employee should be able to upload a client approved timesheet.) c) Once the timesheets are submitted for approval the employee should not be able to edit the timesheets. b) For the accountant login the accountant should be able to view the submitted time-sheets and should be able to approve those. We have to do more changes, depending on how you handle this initial timesheets module. We can plan more changes. Our goal is to customize the admin and the accountant modules.