Within the BIDS environment debugging SSIS packages is not the same ball game as operating with Visual Studio.
If you have created your own custom framework, i.e. from within the visual studio, then you should be able to test the unique features of that component in the Visual Studio environment before making the component usable for use in the BIDS environment.
You can not modify the internal. NET code that makes up any of the other modules built into SSIS (i.e. the FTP task) inside BIDS, and I doubt that there will be any other custom component.
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