Hello i purchased a commercial building post and beam with sip walls and roofs. I have major rot in areas. I also would like to install hardi board siding and am thinking about screwing another layer of ywood to the osb sip panel. Will this be a weight issue?

Jon,
not so much a weight issue as it would be bad practice to bury rotted wood under new plywood/ siding
unfortunately the rotted SIPs need to be removed and replaced