Hi SJ,
If you have got the abap code for converting greg dates to persian dates then all you need to do is to create a customer exit variable and associate the abap code to your variable.
All you have to do is get the input in Persian date form manually and then invoke the abap code that performs the conversion.
Here are a few links that can help you in creating customer exit variables:
Customer Exit Variables in SAP BI - SAP NetWeaver Business Warehouse - SCN Wiki
Hope this helps.
Regards,
Gopal