Access to the special telugu characters such as ardhavisargha, nakarapolla, valapalagilaka, etc, is available through the macro input file itrnstlg.tex. The following table displays the characters available, and their macro names.
Note that ITRANS does not provide support for the udatta and dantyam telugu characters. If you must use these, take a look in itrnstlg.tex -- some examples are given there, but it may require extensive hand-tuning. Examples shown in table 2.
ITRANS Home Page: http://www.aczoom.com/itrans/