You need to have permission to edit dns zone files of all domains under your reseller domain. Normaly there will be persmission.
If you don't have 'Edit dns zone' feature under your DNS functions ,, you need to contact your server owner(support) to get this.
You can also have default nameserver , tell the hosting company to set default nameservers for you.
It is set under Reseller accounts -> select the reseller domain -> edit privileges/nameserver.