hi,
how can we send email to all clients hosted with us using whm/h-sphere?

The function only available for root WHM. While, you should have keep a copy of their email right?

choose billing system like client exec,it will be very useful for u

I would use a billing software to manage the communication side. Not really good to use internal email system build in WHM.

you can also use a 3rd party maling company such as constant contact or you can buy a mass maling software such as mailloop to stay in touch with your clients