If you are hoping for a "film look," there is much more to it, than just the 24p. This is a good tutorial on what is involved: http://www.learningdslrvideo.com/film-look-dslr-video/
A true film look does start with 24p, but goes into lighting, lens selections, etc. as well.
As for converting, say 30i to 24p, I do not know of any plug-in that will do a good job, but there might be some good ones out there.