stm32f7-discovery board. The board is missing from selection of STM32CubeIDE (Ver 1.2.0).

According to me board STM32746G-EVAL is the one you're looking for (STM32F746NGHx soldered on). Such board is highlited as NRND (Not Recommended for New Design means obsolete soon) but still supported. I'm STM32CubeIDE 1.2.0 based.

Is there a known way to force STM32CubeIDE 1.2.0 to enable designs based on "Not Recommended for New Design" boards?

At customers own risk, of course.

Still according what i'm able to do as STM32CubeIDE end user​ NRND is allowing to work with this target. Just sharing such may phase out in some near future.

Is there a way to make it appear in the list if it is not there?

1.2.0 is also missing NUCLEO-F439ZI which is still "Active" so I think the NRND isn't the cause of the OPs board being missing. A related question would be - are there any resources explaining how to add new boards to the IDE? Any place where one could download pre-done definitions from? Any charts showing boards that are roughly equivalent for IDE purposes?

This doesn't help the OP but for people finding this in a search - it looks like you can use NUCLEO-F429ZI as a substitute for NUCLEO-F439ZI as the only difference I can see is the addition of hardware accelerated crypto.