The two major ways I have read people are doing this are;
1. Use a placeholder file, which for a DVD could be a "Video_TS" file, and for a Blu-ray an "index.bluray" file, then run the "Get Movie and TV Info" function, searching for the correct movie, and add the correct metadata to this dummy file.
For example, copy any VIDEO_TS file to a movie specific directory, such as "\Stoker (2013)\VIDEO_TS\VIDEO_TS.dvd;1", and then add metadata to it for the correct movie. For Blu-ray, an example would be "\Alien_ Covenant (2017)\BDMV\index.bluray;1".
But then if you tried to play it, MC would fail. I can't remember how people managed that, but a search of the forum could find the threads discussing the idea. Or just try it.
ErikN's idea would work similarly, but if using a JPG as a dummy file you would need to set the [Media Type] tag to "Video" and the [Media Sub Type] tag to "Movie" so that it displayed in the correct Views. This is a bit tricky unless you have a View that shows all file types so that you can easily locate the files, and have a custom tagging window definition (template) that displays both the [Media Type] and the [Media Sub Type] tags in the one template. I tested and had to go the the Images File View to change the [Media Type] tag to "Video", and then to Video File View to change the [Media Sub Type] tag to "Movie". Not too onerous, but a bit of work if you are doing many at once.
When you tried to play the movie, MC would use the file association with the JPG extension to correctly play the image, which in MC means displays it, so you would get the message.
2. Just import the movie from the optical disc, and make sure that you have the Auto Import Option "fix Broken Links" set to "Yes (protect file on missing drives)", so that MC doesn't delete the Library record when you remove the disk.
No need to change the [Media Type] and [Media Sub Type] tags, as they will already be correct.
The "Get Movie and TV Info" function still works, with the Cover Art saved into the \Cover Art\Videos\ directory instead of next to the video file. (Because it can't save to the optical disc of course.)
Metadata still works, but I don't know where the metadata is stored, or whether it comes from the disc. You can edit some metadata, such as [Name], but doing so doesn't seem to update the Cover Art file name, which it should. Also, if metadata is stored in a Sidecar file somewhere, it may be named "index.bluray_Sidecar.xml" and stored in a common location, which would mean it would get overwritten when you next imported an optical disc. Something to check and get around if it is a problem. The metadata for a disc imported this way it stored in the CD Database in MC, so there is no issue with a Sidecar file being overwritten.
When you try to play you get a message from MC saying it can't find the volume to play it, and asking whether you want to try and play again or cancel. If this is an issue, you could edit the Cover Art image to add a note such as "Optical Disc Required" or similar, such as ErikN suggested above. Just make sure you don't update the Cover Art in future or you will overwrite your modified image. Or write protect the image.
I prefer the second option, but I haven't actually implemented it yet, as I am ripping most discs at the moment, and haven't bothered to go back through my collection to add the older discs.
Anyway, you should be able to find long threads about how to do this on the forum. It has been discussed at length.