If you want to add static routes in DHCP Clients use DHCP option 121 in DHCP SERVER.
It is defined in
http://www.faqs.org/rfcs/rfc3442.html. The option code is 121.
However, Microsoft DHCP server uses another option i.e. 249 to serve the same purpose.
In Linux based DHCP server i.e. ISC DHCP server , you can try adding the options as specified in