Index: cms/sources.py
===================================================================
--- a/cms/sources.py
+++ b/cms/sources.py
@@ -298,8 +298,118 @@
                     result.append(relpath + filename)
                 elif os.path.isdir(path):
                     do_list(path, relpath + filename + '/')
         do_list(self.get_path(subdir), '')
         return result
 
     def get_cache_dir(self):
         return os.path.join(self._dir, 'cache')
+
+
+class MultiSource(Source):
+    """A source that combines the contents of multiple other sources."""
+
+    def __init__(self, base_sources):
+        self._bases = base_sources
+
+    @property
+    def version(self):
+        return self._bases[0].version
+
+    def get_cache_dir(self):
+        return self._bases[0].get_cache_dir()
+
+    def __enter__(self):
+        for base in self._bases:
+            base.__enter__()
+        return self
+
+    def __exit__(self, exc_type, exc_value, tb):
+        return any(base.__exit__(exc_type, exc_value, tb)
+                   for base in self._bases)
+
+    def close(self):
+        for base in self._bases:
+            base.close()
+
+    def has_file(self, filename):
+        for base in self._bases:
+            if base.has_file(filename):
+                return True
+        return False
+
+    def read_file(self, filename, binary=False):
+        for base in self._bases:
+            if base.has_file(filename):
+                return base.read_file(filename, binary)
+        raise KeyError('File not found {}'.format(filename))
+
+    def list_files(self, subdir):
+        files = set()
+        for base in self._bases:
+            files.update(base.list_files(subdir))
+        return sorted(files)
+
+
+def _memoize(func):
+    """Cache results of functions calls."""
+    memoized = {}
+
+    def wrapper(*args):
+        try:
+            return memoized[args]
+        except KeyError:
+            return memoized.setdefault(args, func(*args))
+    wrapper.cache_clear = memoized.clear
+    return wrapper
+
+
+def create_source(path, static=False, revision='default'):
+    """Create a source from path and optional revision.
+
+    `static` flag determines the type of the source that will be created and
+    its caching behavior:
+    - If `static` is `True`, we use version-control-aware `MercurialSource`,
+      pass the revision and cache the results of most used functions.
+    - If `static` is `False` (as is the case with the preview server), we use
+      simpler `FileSource` and don't cache anything to be able to respond to
+      changes on the filesystem.
+
+    If `settings.ini` in the source contains `[paths]` section with an
+    `additional-paths` key that contains the list of additional root folders,
+    `MultiSource` will be instantiated and its bases will be the original
+    source plus an additional source for each additional root folder.
+    `MultiSource` looks up files in its base sources in the order they are
+    provided, so the files in the additional folders will only be used if the
+    original source doesn't contain that file.
+    """
+    if static:
+        source = MercurialSource(path, revision)
+    else:
+        source = FileSource(path)
+
+    config = source.read_config()
+    try:
+        ap = config.get('paths', 'additional-paths').strip()
+        additional_paths = filter(None, ap.split())
+    except ConfigParser.NoSectionError:
+        additional_paths = []
+
+    if additional_paths:
+        additional_sources = [
+            create_source(os.path.join(path, p))
+            for p in additional_paths
+        ]
+        source = MultiSource([source] + additional_sources)
+
+    if static:
+        for fname in [
+            'resolve_link',
+            'read_config',
+            'read_template',
+            'read_locale',
+            'read_include',
+            'exec_file',
+        ]:
+            setattr(source, fname, _memoize(getattr(source, fname)))
+
+    return source