I normally don't bother adding the routes until 90% of the nodes and connections are done, and the principle actions are in place and tested, Some small maps can be played with no routes since the bots can 'see' everything and normal roam actions (#12) are enough to get them to play the map thoroughly.
When you are at the point to add routes, identify the locations in the map that the bots will obey routes (spawn point, an alt roam for another route, or an objective task upon completion). In atlantic wall the allies spawn can have a route to direct some engineers to the dynamite wall location, some to the command post, and other bots to camp positions, fixed mg42 camps and the ladder opening. Once the wall is blown activate objectives inside the fort either using the goal_tracker or by using action_groupings activated in the aiscript. The goal tracker works well for largely linear play where no more than 2 dynamite objectives will have the same goal number and where there are no constructable impediments to navigation.
Anyway read the wiki page on The route system. It is a good piece of work. I am throwing together a brief l
inks page on my site for those who don't like to go linearly through the manual and tutorials.
(If you are asking about adding a secondary route at the altroam end of a master route, you can add more than one route if there are so many directions and actions to support. I think the start of the secondary route should be overlapping a bit of the altroam of the master route. To be certain about your routes you need to do a bit of testing including isolated route testing.)