How can I steam vegetables in my rice cooker without a steam basket?

It's extremely difficult and you can run into problems. May I suggest you either get a 2nd steamer or get the one I have that has 2 layers. I bought it from Sears last month only $34.99. It's a Black & Decker model and is awesome, very easy to clean and has a timer that goes off when done. There are others that have 3 levels, whatever suits your needs.

When you steam buns in a rice cooker / food steamer you can use paper towels, however you will not want to line the steam tray. What you will want to do is wet the paper towel and then lightly squeeze out excess any excess water. Wrap the bun in the damp paper towel and place it in the steam tray. Wrapping the bun in a damp paper towel will allow it to be steamed in the rice cooker without it sticking to the paper towel.