Extend DHCP range same subnet

Hi,

My network is 192.168.1.0/24

DHCP range 192.168.1.51-100

Server range 192.168.1.101-120.

I need 40 additional ips and  I have free ip from 192.168.1.121-160,

but I cannot extend the DHCP range.

I  am getting following error.

"The address range and mask conflict with an existing scope"

I was wondering how can I add additional 40 ips in DHCP?

Hope it is clear.