Is it possible to add a driver to my kernel without flashing a new kernel? Is it possible to load a driver as a kernel module into my Galaxy S10, Magisk rooted without having to build a new kernel and flash it?
Drivers are built as kernel modules i.e. .ko files which can be loaded with modprobe but for that you have to rebuild kernel with the required module, or it should be compatible with installed kernel if built separately.

The installed kernel should be built with CONFIG_MODULES=y in order to load modules. Make sure the version of kernel source code tree or that of header files match with installed kernel version. Otherwise dependencies may break resulting in Unknown symbol in module type of errors. Also, if kernel is signed (CONFIG_MODULE_SIG_FORCE), modules must be signed with the same key to avoid required keys not available error.