Probably the reason why it is not added is because it wasn't considered "one of the main ones" whatever that means. You can write ILS yourself though: you can use whatever three letter code you like.
Alice trusts Bob for 10 XYZ.
Alice and Bob know what XYZ represent. If you and your friends (ripple neighbors) use ILS, you know it represents New Israeli Shekels, the system only needs to know that it is the same currency so that it can apply a 1:1 exchange rate.
For more information on this, read this issue and this forum post:
EDIT: To trade ILS, manually edit the text within the selector for the currency pair. For example, substitute "XRP/BTC" for "BTC/ILS" and that's it. Of course you need to specify the issuer for ILS and BTC as always.
EDIT2: Now currencies will be ordered upon usage. The more you use a currency, the higher it will appear on the selector's list.