nested if expressions for string in excel source file in derived column expression I have a column named Product which contains strings.

Everything starting with "ZYT" should be shown: "ZYTOL"
and "AZ" --> "AZAB"
    "UXO" --> "UXOL"
    "ADX" --> "ADXIZN"  

How could I write the expression in SSIS transforming process?
Your expression will probably look something like this, in a derived column transformation.

(LEFT(Product, 3) == "ZYT" ? "ZYTOL" : 
    (LEFT(Product, 2) == "AZ" ? "AZAB" :
        (LEFT(Product, 2) == "UXO" ? "UXOL" :
            (LEFT(Product, 3) == "ADX" ? "ADXIZN" : Product))))


Please note that depending upon your data type, you may need to cast some of your strings to Unicode/non-Unicode types to get everything to work smoothly. 

The syntax uses the ternary operator https://msdn.microsoft.com/en-us/library/ms141680.aspx