You can setting variables in a user directory entry (<variable> entries in <variables> section). Examples are on the Wiki.
They'll be set for incoming calls and would affect the behaviour.
If you're wanting to set them on the outbound channels based on destination, I'd suggest configuring the destinations as sofia gateways. In the gateway definitions you can set variables to be set on incoming/outgoing/both calls via that gateway... setting outbound variables there should affect the bleg. Again, examples on the wiki.