There's no such thing as 25i - do you mean 50i?
Some NLEs will let you edit different frame rates in the same timeline, others won't - I'd still rather have all footage in a common timebase before editing to avoid problems further down the line.
Does the 50i footage need to be in sync with anything else - audio tracks or other camera angles?
If you deinterlace to 25p and then conform to 24p, you can probably get away with it. The exact steps you follow will depend on your editing software.
I'll let Alcove chime on what to do if you need to sync audio with the 50i footage…
(Thought it was worth mentioning that pretty much any film exhibited internationally will be shot at 24p and then converted to 25p when published on DVD in the UK - a speed increase of 4%, a difference which people obviously don't notice.)