Firstly Boost's offerings are not Caps (like Telstra Pre-Paid).
If you wish to purchase an Add On for something like Premium SMS sending, calling overseas, calling from overseas or extra data, you can purchase an Add On at anytime. Purchasing an Add On in no way affects your UNLTD recharge.
Simply go to the recharge option on http://m.telstra.com/boost on your handset and after ou select your recharge method, there the three UNLTD recharge options is the Add On options $5, $15 and $25 with links to the info pertaining to what you get for your money.
remember recharge only the ADDON if you have run out of data from your UNLTD recharge.
I wanted more data and paid $15. It has not been added to my existing data...... It is showing as 100 minutes of international calls.
If you are looking in the app, open up the "more info" section. The addon data could be in a different field.
* I am a Boost Australia customer. This is a peer to peer community where users help each other.