How to call a "sub" .inputrc? (Like in .bashrc, where you can "source sub_script.sh")

According to man readline:

$include

This directive takes a single filename as an argument and reads commands and bindings from that file. For example, the following directive would read /etc/inputrc:

$include  /etc/inputrc

If you only want to change the location of the file, you can set INPUTRC to my_config/my_actual_inputrc (if unset, defaults to ~/.inputrc).

There is also an equivalent of source, that is $include (source). For example:

$include myconfig/my_actual_inputrc

Another option is to use a link:

ln -s my_config/my_inputrc .inputrc

That will create the file .inputrc as a link pointing to my_config/my_inputrc.