Hi Sagar
If it is DP then the forecast can be extracted from DP Planning Area to an Infocube. From the Infocube you can generate flat files easily using Infospokes (Open Hub Service).
These flat files can then be FTPed from the APO Application Server to your third party system for usage.
If you are also have SNP then you can also think about using /sapapo/bp1 to transfer the order series data.
Please let us know if the above information would be helpful to you.
Thanks
Amol