You need to add them to BARN or just make a new MCD set, otherwise if you use that one to place the vanilla CULTIVAT tileset it will completely mess up the existing maps.
Can you please check if I understand your meaning correctly ?
A map (-block?) relies on a ordered list of MCD files
example :
1) uses A.MCD, 10 entries
2) uses B.MCD, 15 entries
Then map will refer to individual tiles by index 0-24 : 0-9 are those of A.MCD, 10-24 are those of B.MCD
-> Adding a tile in A.MCD, even "at the end", breaks the map because indexes 10 starts referring to this new tile instead of "first tile of B.MCD", etc.
Adding tiles 'only in MCDs which are only used last' sounds flaky, so I guess a rule of thumb is to not add, insert, delete or renumber any tiles to a MCD after it's released publically, unless you're sure that you can adapt every map that uses it.