You can, of course, delete the hostset and then re-add it:
cr_cli -cmd hdb.hs.del -ID NM:my-hostset -u admin -p admin
cr_cli -cmd hdb.hs.add -name my-hostset -desc "Hosts which are mine" -hIDs
NM:host1,NM:host2 -u admin -p admin
or, to use hdb.mod instead of del/add:
cr_cli -cmd hdb.hs.mod -ID NM:my-hostset -hIDs NM:host1,NM:host2 -u admin -p admin
The trick with the mod command is that you have to list the complete set of hosts you want to exist in the hostset when you're done, not just the new ones. Any argument you omit from the mod command is unchanged.