so you have one attribute to add to a lot of products?
the only way it changes from a radio to a drop down is if you have more than one attribute listed.


if you want to add the attribute(s) to a few products you have to go up to the "attributes controller", find a product that already has the attribute, either click on the "copy attribute to product" or "copy attribute to category" button and (i choose this) click on "ignore and only add new attributes" button, then "copy" button.
ive done this alot of times and it saves me time and trouble.