There are a couple of pieces to this.
First: Your route-map.
The route-map needs the matching conditions. So, you would create an access-list with the routes you want to prepend and then another to allow either all other routes or just the ones you want to use without being prepended.
An example for this:
router bgp 65100
network 172.16.0.0
network 172.17.0.0
neighbor 10.69.232.70 remote-as 65200
neighbor 10.69.232.70 route-map set-as-path out
!
route-map set-as-path 10 permit
match address 1
set as-path prepend 65100 65100
!
route-map set-as-path 20 permit
match address 2
!
access-list 1 permit 172.16.0.0 0.0.255.255
access-list 1 permit 172.17.0.0 0.0.255.255
!
access-list 2 permit 0.0.0.0 255.255.255.255
So in this example, the route-map will only prepend 172.16.0.0/16 and 172.17.0.0/16 routes but still allows all routes.
Second: The no-prepend option in the local-as configuration does preclude any route-map prepending you do. It stops the routes received from the eBGP Neighbor from being prepended by your local ASN.
So, yes, you can still prepend with the route-map even with the no-prepend option in the local-as config.
Here are some references you can use for further research if you wish:
Cisco guide for BGP Local-AS
Cisco Guide for IP Route Filtering in BGP
Cisco Guide for DUAL AS Configuration
While these guides are not exactly for your equipment, the information about the commands, their usages and most syntax are still applicable.
I hope this helps!
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