How would you do that manually? Maybe with following steps:

1. Select the word.
2. Copy it to clipboard.
3. Open the file with the list of words.
4. Go to bottom of the file.
5. Paste the word.
6. Insert a line break.
7. Close the list file with save.

Exactly this can be done also with a macro:

IfSel
Clipboard 9
Copy
Open "name of list file with full path"
Bottom
IfColNum 1
Else
"
"
EndIf
Paste
"
"
CloseFile Save
ClearClipboard
Clipboard 0
EndIf


What you want to do could be also done faster without a macro by using a user tool.

Open Advanced - Tool Configuration and create a new tool with following settings:

Tab Command
Menu Item Name: Add selection to list file
Command Line: echo %sel% >>"name of list file with full path"
Working Directory: select with the Browse button the direcotry of the list file - please really use the Browse button
Toolbar bitmap/icon (file path): whatever you like and have

Tab Options
Program Type: Dos Program
Save Active File: unchecked
Save all files first: unchecked

Tab Output
Command Output: Append to Existing
Show DOS Box: unchecked
Capture Output: unchecked
Replace selected text with: No Replace

Okay, now you have a tool which you can run whenever you want to append to your list file a line with the current selection.

You can assign a hotkey for the user tool and you can customize the toolbar and add the user tool to the toolbar.

Note: I have both methods not tested.