Try a ratio calculation of frames (24 subtitle fps in PAL for 25 fps) and apply that ratio to 29 frames for 30 fps NTSC (29.97 really, but who's counting) and round it to the nearest frame. You should be able to do something like that in Excel with a formula. There might even be a template out there on the web somewhere that does this already.
You might also want to look into a dedicated subtitling application - Lemony or others. They may have a function that does this for you. If such features exist, this could be the most painless method.
The lazy/poor man's way is to just ignore the differences, which guarantees your subtitles will be not frame accurate (off by no more than 2/15ths of a second), but the real risk in this approach is the duration of your subtitles might crash into the next subtitle, which will cause an error condition when you import the script into Encore (you'll lose the offending subtitle clip). This might work for you anyway, or not.
Thanks for you help. Finally I just imported the PAL subtitles, they lined up just fine, sometimes with a few frames difference. I repositioned them according to the PAL timecode and now it looks fine! No need doing timeconsuming conversion.