I would use a program like DV film maker to convert your 60i footage to 24p. It all really depends on what you want your end result to be. If you want a 24p DVD then that would be the way to do it. However, you can keep the 24p look and edit in a 60i timeline, which would then keep both your 24p footage and your 60i footage looking its best.
You mean you want to put 60i material, running at full 60i speed, in your 24P project?
Withoutit trying to convert to 24 frames per second, right?
Or are you trying to put 60i in a 24P timeline and use it as slow-motion?
If you intend to edit 24P and 60i footage together, the way to do that is on a 60i timeline in the first place.
If you've already edited it in 24P, and now you need to add 60i to it (and keep the 60i look), then try just changing your project preset to 60i instead of 24P... it should keep all the edits and everything, and allow you to put 60i in the program.