The next major Drone release will support sourcing the yaml file from a plugin. Drone will make an API request to your plugin, at which point the plugin is responsible for returning a valid Yaml file back to Drone. This means you could create a plugin that dynamically assembles your Yaml from multiple pieces.
So the answer to your question is, yes, it will be possible, but since it is not something we are going to support natively in Drone it means you will need to write a plugin, or wait until someone else writes a plugin.