It is a common situation that some project allows for multiple alternative licenses, some of them are "free" and expressible via SPDX, while others of them are proprietary.
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As the free licenses are always a legitimate choice for the users and redistributors of those packages, I propose that to this express via a special (reserved) identifier "CustomAlternateLicensing".
In a SPDX license expression you could use "OTHER", e.g.: (AGPL-3.0-only OR OTHER). That's less specific, but it does provide *some* information, it and doesn't require that a SPDX document exist.
As Sam Ellis noted, if you use a SPDX document you could name custom licenses and that would completely resolve the issue. However, that will only help you if you use SPDX documents. (As I've noted elsewhere, I think most SPDX users only use SPDX license expressions, and do not create or use full SPDX documents.)
