Attach Gallery Images

I’m putting together a page for a client so i don’t have a public URL to share, but was wondering if there’s another way to attach/add images to the Gallery without filtering to “uploaded to this post”.

currently my steps are:
1. Click “Add Media” button, above the Editor area/below the Title
2. Filter to “Uploaded to this Post”, which is empty (this is the part i have trouble with as I can upload new images that get automatically attached to the property post, but can’t select from the media gallery AND have it attach to the post)
3. Click the Upload button; select the images from my computer
4. Instead of clicking “insert into post”, just click the “x” close button
5. Click Update, to update the property listing.

What if the images are already uploaded to the media gallery? Can these images be attached to the property post?