ede-auto-add-method

ede-auto-add-method is a variable defined in `ede.el'.
Its value is

ask

Documentation:
Whether a new source file should be automatically added to a target.
Whenever a new file is encountered in a directory controlled by a
project file, all targets are queried to see if it should be added.
If the value is 'always, then the new file is added to the first
target encountered. If the value is 'multi-ask, then if more than one
target wants the file, the user is asked. If only one target wants
the file, then it is automatically added to that target. If the
value is 'ask, then the user is always asked, unless there is no
target willing to take the file. 'never means never perform the check.

You can customize this variable.