You don't say which version of the program you're using -- but you should be able to mix video sources in any version of the program after version 11.
However, the video you add will be conformed to your project's settings. In other words, if your project is set up for 30p video, then adding 60p video will net you 30p video. It won't play individual frames to make your video 1/2 speed.
On the other hand, the program does include an excellent tool (Time Remapping) for changing your playback speed to fast or slo mo.
As always thank you Steve..............where exactly are the options for a project's settings?
When you start a new project, the Settings button can be found on the New Project panel.
Otherwise, in any version of the program after version 11, the program will automatically set your project settings based on the first clip on your timeline.