Program to convert videos by interpolating 30fps video to 60 fps similar to Splash Pro video player
If you're exporting your finished project to a 60 fps format, your editing or compressing software will generally do some sort of conversion for you. It's not usually necessary to conform all your clips before editing.

If you have different options for how to handle the conversion, using "Optical Flow" or "Motion Estimation" options will interpolate frames. Depending on how good the interpolation is and the nature of the footage, this may look perfectly acceptable or horrible. As mentioned, you can try frame blending, too.

If your software doesn't do the conform automatically, you could try 3rd party tools. I know many people who have had good success with Twixtor from RE:Vision Effects. But there are others you can try as well depending on which application you're using.