You can configure the ASN of the autonomous system in which the remote neighbor resides.
device# configure terminal
device(config)# router bgp
device(config-bgp-router)# local-as 1000
device(config-bgp-router)# neighbor 10.1.1.1 remote-as 1001
device# configure terminal device(config)# router bgp device(config-bgp-router)# local-as 1000 device(config-bgp-router)# neighbor 10.1.1.1 remote-as 1001