Server Side Plugin, how to add one plugin dependencies into another plugin ?

It is unclear what you try to do.

It depends. Look at the EWM Extensions Workshop. The follow up action consists of a common and a server portion. 
This common plugin is deployed with the extension feature. The server plugin has a required plugin dependency in the plugin.xml. The deploy code is generated from the feature that references both plugins in the update site. Look at the final structure of the Extensions Workshop and the deployment steps.

If you have a common plugin, that is supposed to be used by multiple extensions, the best approach I am aware of, would be to create a feature for the common plugin and generate deploy code for it. Then add a dependency to the feature/plugin in the code that is supposed to use it. You would either have to deploy it in the debug server, for this to work, or you import the plugin and feature code into your workspace.