First, regarding internships. These will tend to be experiences developed and filled on a local basis, consequently other than ACHE's own summer diversity internship or the Institue for Diversity's Summer Enrichment Program, such opportunities do not come to our attention at the home base of ACHE. You should explore opportunities for experiential learning (that's more than just internships....case studies, job shadowing, etc) in association with participating in your local ACHE chapter.
As for credentials, ACHE offers only the FACHE which is earned after passing the Board of Governors examiniation. Often, though, ACHE course work will count toward satisfybing continuing education requirements of numerous licensing or credentialing agencies. That is just for clarification.
My hunch is that you would be best served employability-wise were you able to pursue your interest in Human Resources management as a functional specialty within your MBA program. I am sure you can find rleated courses or publications from ACHE, Health Administation Press or elsewhere that would add a valuable aspect of HR in contemporary practice to your studies.