Can gtk2 themes be converted to gtk3 themes?

Ported well, Yes and no.

GTK 2 Themes make use of a gtkrc and other files like .png images.

GTK 3 Themes make use of CSS for theming.

CSS was chosen because it was easier to work with to make theming less difficult, so you would only need to create the needed CSS files to match the Colors and gradients, etc. of the GTK 2 you want.

So there's really nothing to port.

There is no tool that will take a gtk 2 theme and convert it to a gtk 3 theme.

The gnome documentation describes how to migrate a theme

http://developer.gnome.org/gtk3/stable/migrating.html

Part of the guide on theming

http://developer.gnome.org/gtk3/stable/

The only tool I know is gnomecc 2 gtk3css converter. It converts a Gnome Color Chooser color scheme into a GTK3 CSS file.