Eclipse: Create a linked source folder
With the GUI...
Right click the project then
Build Path > Configure Build Path > Source > Link
The "remote" folder needs to have "src" folder which you include
And the "locate" folder does not have "src"
Without the GUI...
Close your project.
Add it to your .classpath file (in the root folder of your project)
<?xml version="1.0" encoding="UTF-8"?>
<classpathentry kind="src" path="mylinkedfolder"/>
Add it to your .project file (in the root folder of your project)
Open your project and refresh (F5) your folders.