If I understand you correctly it can be done with an offset-list command.
eg. offset-list 10 in/out 16
Sets metric to 16 for routes that match access-list 10.
or
(not exactly what you asked but close) Under rip change the admin distance. Using the distance command in the routing process subconfiguration mode you can also change the distance for routes learned from one source only in some protocols, and you can change the distance on just some routes. See this link:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_r/iprprt2/1rdindep.htm#1017506
Hope it helps
Steve