how to send a predefined path to a local planner?

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I guess the best way is to write your own global planner plugin, which adheres to the respective interface there. However, this then does nothing when receiving the usual move_base goals.

Rather, you add whatever ROS concept you want (subscriber, service server, action server) to this global planner plugin for retrieving the path from wherever you want (publisher, service client, action client) and pass it down the chain.

Then, you can use the full move_base structure and can have deterministic global plans...

The path generated by the standard move_base mechanics is stored in a topic (I think it's called just path) composed of an array of vectors. My point is, those vectors are the same kind of vectors that describe the actual pose of the robot, so maybe you can build a node that retrieves the pose of the robot (taken from the odometry or whichever mechanism you're using for the localization) while you move manually your robot in the trajectory that suits your purposes.

That way you'll have an array of vectors describing the path you just made with your robot, which with minor adjustments you can publish into the path topic of move_base.

I don't know how complex this could be, but if it isn't possible the easy way is record a bag file of a path generated from move_base that seems good to you, and replay it anytime you want.

A more recent solution to this problem is to use move_base_flex. You can use the exe_path action and specify a custom path and your desired local planner (path tracker) to follow that custom path.