The following thread, which is entitled “High Plane Unicode Mapping”, may be of interest in relation to learning how to add characters that are mapped to one of the planes beyond the Basic Multilingual Plane (that is, beyond plane 0).
The thread includes a post from Erwin Denissen that showed me how to proceed.
There is also the thread entitled “Font with some Emoji in a high plane Private Use Area” that refers to the above-linked thread.
Thus far that thread only contains posts from me, though I am pleased to note that the various files that I have uploaded have been downloaded a good number of times, though one download for each being my provenance checking of the upload.
A related thread that is well worth studying is “Obtaining a hexadecimal dump of a Unicode Text Document”.
In that thread Erwin kindly uploaded a utility program. This is an extremely useful program.