Nested hostgroups

Does anyone know how I can add a hostgroup into a hostgroup (nested)

example Suppose I want A hostgroup Belgium and in this group all windows servers (hostgroup) The last one contains the windows hosts.

define in hostgroups.cfg
define hostgroup{
hostgroup_name Belgium
alias Belgium
contact_groups Belgium
members host1, host2, host3
}

define hostgroup{
hostgroup_name windows_servers
alias windows servers
contact_groups windows
members host1, host2, host3
}