1. Create a file list named something like: yourlist.txt. Add words, one per line, to yourlist.txt.

Save your file to a location like this depending on where you installed Scrivener:
c:\Program files\Scrivener\Aspell\bin\yourlist.txt

Your list might look like:-

malondialdehyde
neuropsychology
Silybin
vagus


2. Convert file list to an Aspell word list.

This command creates an Aspell wordlist with the en_US language specification (en_GB is another example which you could use):

Move to the following directory with a command prompt:
cd c:\Program files\Scrivener\Aspell\bin

Type the following command and then press Enter:
aspell --lang=en_US create master yourlist.rws < yourlist.txt



3. To finish adding it to the dictionary change into the dictionary directory by running the following:

c:\Program files\Scrivener\Aspell\dict


Then edit the following file with a program like Notepad++: en_US.multi
Note: Windows Notepad will not work. Notepad++ is a free download and a great tool.


Then add the following on a new line at the end of the file: 

add yourlist.rws

Restart Scrivener and your words will now be updated in the main Aspell dictionary