diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -178,6 +178,12 @@
             _(b'add single file all revs overwrite'),
         ),
         (b'n', b'new-file', None, _(b'add new file at each rev')),
+        (
+            b'',
+            b'from-existing',
+            None,
+            _(b'continue from a non-empty repository'),
+        ),
     ],
     _(b'[OPTION]... [TEXT]'),
 )
@@ -188,6 +194,7 @@
     mergeable_file=False,
     overwritten_file=False,
     new_file=False,
+    from_existing=False,
 ):
     """builds a repo with a given DAG from scratch in the current empty repo
 
@@ -226,7 +233,7 @@
         text = ui.fin.read()
 
     cl = repo.changelog
-    if len(cl) > 0:
+    if len(cl) > 0 and not from_existing:
         raise error.Abort(_(b'repository is not empty'))
 
     # determine number of revs in DAG
diff --git a/tests/test-completion.t b/tests/test-completion.t
--- a/tests/test-completion.t
+++ b/tests/test-completion.t
@@ -272,7 +272,7 @@
   debugantivirusrunning: 
   debugapplystreamclonebundle: 
   debugbackupbundle: recover, patch, git, limit, no-merges, stat, graph, style, template
-  debugbuilddag: mergeable-file, overwritten-file, new-file
+  debugbuilddag: mergeable-file, overwritten-file, new-file, from-existing
   debugbundle: all, part-type, spec
   debugcapabilities: 
   debugchangedfiles: compute