Dear atin83 -
If you are speaking strictly about HACCP - not in the context of GFSI or another certification body - then 'yes', I have used an outside expert. Given in the US that HACCP is more regulatory than scientifically based, I chose someone with extensive experience in my products & processing procedures. This particular person also has a broad knowledge base in multiple processing areas, so was able to bring in ideas outside of exactly what I do.
I do not normally evaluate consultants primarily on their certifications, as I have known people who can pass just about any exam, but still be woefully ignorant of real world practices and void of common sense. That is not to say that certifcations do not have their place...
Always ask for references - particularly for past consulations - and actually check them out.
Keith