Have a look at below link.

http://www.ssas-info.com/analysis-services-faq/27-mdx/2076-q-i-have-measure-in-my-cube-that-shows-duration-on-some-event-in-seconds-how-can-i-make-result-appear-in-the-format-hhmmss

Ignore if you already pass through this.

Thanks,

Ashish