Just dropping the footage into the timeline will play it at original speed, but using Command+R you can open the retime bar and drag the edge of the clip to slow it down to a desired percentage. I guess you would want to slow it to 24 / 60 = 40% speed.
I am not sure if there is an option to guarantee that 1 original frame is matched to 1 frame in the output, which is what I would want ideally. I guess you sort of have to hope that you’re slowing it down to the right amount.
I’ve had a lot of problems with slo-mo videos and inaccurate retiming in FCPX. Sometimes retiming seems to go jittery on clips for no reason at all. The 10.0.1 update does include a fix for retiming, but I don’t know if it fixes my issue. If there is jitter in the resulting output, a stop-gap measure might be to use the “optical flow” algorithm for rate conforming.