You can do this and it is quite easy to do. However you must have the ISP do this as well. Here is a link I found that may help:
http://www.cisco.com/en/US/docs/interfaces_modules/shared_port_adapters/configuration/7600series/76cfgt1.html#wp1192082
Not specific for your router but the info is still the same.
Here is a sample config too:
interface Multilink1
bandwidth 3072
ip address
ip access-group Multilink1_in in
no keepalive
no cdp enable
ppp chap hostname
ppp multilink
ppp multilink group 1
ppp multilink fragment disable
!
interface Serial0/0/0
description
bandwidth 1536
no ip address
ip nbar protocol-discovery
ip flow ingress
ip nat outside
ip virtual-reassembly
encapsulation ppp
no ip mroute-cache
load-interval 30
fair-queue
no cdp enable
ppp chap hostname
ppp multilink
ppp multilink group 1
max-reserved-bandwidth 100
!
interface Serial0/1/0
bandwidth 1536
no ip address
ip nbar protocol-discovery
ip flow ingress
ip nat outside
ip virtual-reassembly
encapsulation ppp
no ip mroute-cache
load-interval 30
fair-queue
no cdp enable
ppp chap hostname
ppp multilink
ppp multilink group 1
max-reserved-bandwidth 100
!
ip route 0.0.0.0 0.0.0.0
