You should be able to select the network button and enter a IP address range with mask.
example for a network range where the 1st IP was 192.168.0.96 and using a mask of 255.255.255.224 this would cover the IP range from 192.168.0.96 to 192.168.0.223
The LTM help screen states for Destination:
Specifies destination IP address information for the virtual server. You can select Host or Network, depending on the type of virtual server you are creating.
Host: Specifies that the IP address you type represents a single host address.
Network: Specifies that the IP address you type is a network address, which the system uses in combination with Mask to represent a range of IP addresses.
Address: Type the IP address of the destination you want to add to the Destination list.
Mask: If you are adding a network address, type the mask that represents the range. You must type the full netmask. For example, you can type ffff:ffff:ffff:ffff:0000:0000:0000:0000 or ffff:ffff:ffff:ffff:: (with two colons at the end). This field does not support specifying the prefix length in bits, for example, 0000:0000:0000:0000/24.