Make a 59.94 fps sequence.
As you've seen when you enter the 60fps material in a 30fps sequence, Premiere automatically converts the built-in material by adjusting it to the sequence.
To do what you want, I teach you a trick that I did in old versions until the CS6, since at that time to pass a video from 60 to 30fps was a disaster.
In the project panel, right click on the video modify> Interpret footage, in Assume this frame rate you set the value that corresponds to the sequence that you intend to incorporate, in your case 30 and then you incorporate it into the timeline, or you stretch it if It is already incorporated.
Keep in mind that the speed will drop almost 50%, this I use for slow cameras although in the CC versions works quite well.