Obviously you are listing with the quick listing tool. You can revise your listing to use the advanced tool without ending your listing. Open your current listing by using the revise option. Up the top, click on change category. On the next screen, right up the top right is switch to advanced listing tool. Click on that, ignore the warning box that pops up and select the switch button.

On the next screen, you will have an empty box and above it, a blue link to your listing. Click on that and it will open your listing in the advanced tool. You can then add another category by clicking on the change category at the top of the screen.

By using the advanced tool, you can also add extra item specifics and even add a page view counter to the bottom of your listing.

Just remember, listing in a second category costs you money and you will be charged that fee everytime an unsold item relists automatically.