diff --git a/core/src/vfs.sld b/core/src/vfs.sld
index bd1d4f4..e09e8f5 100644
--- a/core/src/vfs.sld
+++ b/core/src/vfs.sld
@@ -13,7 +13,7 @@
     vfs-dir-filter vfs-dir-filter-all
     vfs-subdir
     vfs-from-directory vfs-from-store
-    vfs-to-store)
+    vfs-to-store vfs-append-file)
 
   (begin
     (define (read-full-file port)
@@ -132,4 +132,12 @@
         store-path
         (let ((osdir (store-path-realised store-path)))
          ; TODO(puck): use builtin:fetchurl here instead of reimporting (requires divining a hash first)
-         (vfs-from-directory osdir))))))
+         (vfs-from-directory osdir))))
+   
+    ;; Returns a new VFS, with one file added.
+    (define (vfs-append-file vfs path contents)
+      (define split (string-contains-right path "/"))
+      (define dirname (if split (string-copy path 0 split) ""))
+      (define filename (if split (string-copy path (+ 1 split)) path))
+      (make-vfs (mapping-set (vfs-contents vfs) (cons dirname filename) contents)))))
+