Thank you- this is a great help. As a 'for example'- I want to add Japanese Telegraph stamps, which are not listed in Scott.
There are 10 of them. In the Sakura catalog they are listed as TE1 - TE10. So following your recommendation, I would add a new category under Japan for Unlisted- Telegraph Stamps, then I would give the individual fake Scott numbers (keeping them basically same as Sakura for simplication. So Sakura TE8 could be Scott unlTE8 (unl for unlisted). Then of course any detail such as printing, perf, etc. Then add to my collection or want list with album identifiers, page, column, row, etc. And I could manually enter value information from the other catalog (and I think I would enter the year of that catalog printing, since no automatic updates will be coming.
I'm sure I will also do this for my Olympic Games collection, where so many issues came from the Desert Shiekdom countries which Scott never listed. But Michel does, so I do the exact same procedure from the Michel information, giving them fake Scott numbers which will never clash with real Scott numbers.
I am torn what to do with things like Romania souvenir sheets. During the cold war years, they would issued sets and 2 or 3 souvenir sheets, and often Scott only lists one souvenir sheet and "footnotes" the other two. So these are items that Scott does mention, but doesn't assign a Scott number (but sometimes shows a value). For example
A Scott set might be
1202
1203
1205
1205
1206a souvenir sheet - perf - red color
footnote: an imperf souvenir sheet also exists in blue color, value $10.
In a case like this, I'm thinking about adding them as 1206a.01- and go ahead and describe it as Scott did. (all of these are in the Michel catalog, by the way.
This way, the stamp gets "attached" to the same set of Stamps as Romania issued it, and as Scott acknowledges in their footnote, without parking it in a whole separate Category where it would be an orphan.
Any comments, suggestions, or things I've overlooked?