@zaza If you need to save cookies data to same files, you need manually update file:
Save it to list. (Read File To List action)
Find account, that you using. (If action)
Generate line login:pass:cookies. (just add cookies data to that line with Set Variable action)
Update list with that line. (Set Element action)
Save list to file. (Write List To File action)
If you don't have requirment to save everything in one file, you can still use my example, it will work with multiple accounts.