nano M-A hotkey

Solution 1:

M-A means ALT+A. But that does not directly appends the contents to the file selected.

ALT+A toggles the mode of modification of file between "write to file" and "append to file" as shown in the following images.

After you toggle to the desired command, press Enter.
This will append the contents to the file.

Solution 2:

A test example using the append write file output option.
Starting conditions:

doug@s15:~$ cat cccc
this is file cccc.
doug@s15:~$ cat bbbb
this is file bbbb.

Now, nano cccc, and then Ctrl+O, followed by CTRL+X, followed by Esc then A but change this:

File Name to Append to: cccc

To this:

File Name to Append to: bbbb

And then Enter followed by CTRL+X
See this:

doug@s15:~$ cat bbbb
this is file bbbb.
this is file cccc.

Note: There are other key combinations to achieve the same end result. See the help file within nano, a segment extracted below:

 The notation for shortcuts is as follows: Control-key sequences are
 notated with a caret (^) symbol and can be entered either by using the
 Control (Ctrl) key or pressing the Escape (Esc) key twice.  Escape-key
 sequences are notated with the Meta (M-) symbol and can be entered using
 either the Esc, Alt, or Meta key depending on your keyboard setup.