though MS documentation says that Conversion from DT_STR to DT_BYTES is a legal conversion, Nether data conversion or derived column column compopnents seems be doing this right with out throwing a cast error.

http://msdn.microsoft.com/en-us/library/ms141704.aspx

The only way i was able to cast is using the script componenet.

Row.OUPUTCOLUMN = System.Text.Encoding.UTF8.GetBytes(Row.INPUTCOLUMN);

Not sure if your case how far this will be helpful.

Thanks

rm