I should have better read the topic. You want to change nh attribute for a route learnt by iBGP that you want to reflect in iBGP... This is bad in BGP (https://tools.ietf.org/html/rfc4271#section-5.1.3) and could result in network issues. So I hope you know what you're doing. RR shouldn't modify nh attribute for clients and non-clients and I'm surprized JunOS would do that by default (https://tools.ietf.org/html/rfc4456#section-10). I believe a routing policy can allow you to modify the nh attribute, there's a next-hop parameter from what I recall. Check the routing policy chapter in the EXOS User Guide (http://www.extremenetworks.com/support/documentation/), sorry I don't have access to it right now to give the exact syntax.
The eBGP diagram confuses me a lot. Should I understand A and B are in AS 100 with an iBGP session and C is in AS 200? I imagine also the identical /30 is a copy-paste typo. Assuming this, the eBGP scenario is expected, and without nhs on B I imagine the network from C is unreachable from A. I was referring to this for nhs keyword.