Yes - on an interpretationary level it means the same thing: 'for' references the person who will be given a reward because he offered help, and 'to' does the same.
Linguistically, however, they aren't as equal. Whereas 'for' doesn't require explanation with a verb [making the original sentence correct], 'to' does - if you're using 'to', it has to be phrased like this:
'The will be a reward given/donated/offered/etc. to those who offer their help.'
Hope that helps.
Now...since I've offered my help, what about that reward? 