It's possible to add custom vim plugins by using the startPlugins and lua DAG settings. First we install the plugin by adding it to startPlugins. This example uses nvim-surround, but the process will be similar for other plugins as well.
Then we continue by requiring the plugin in lua using DAG settings. Please note that you're able to name this setting to however you want, the name will add a `--SECTION <name>` in the init.vim, under which it will be initialized.
luaConfigRC.nvim-surround = nvim-flake.lib.nvim.dag.entryAnywhere '' # nvim-flake is a reference to the flake. Please change this accordingly to your config.