When I try to add email folders on Lumia 925, no options are made available - there are only the basic folders such as "sent" or "inbox". Could anyone advise on possible solutions?
I'm sorry, but I'm pretty sure that's not possible with the Windows Phone 8.1 email client. I just checked. Depending on your email provider, you may be able to go to their website on your phone and add it there. I was able to do this on my WP8.1 in Outlook.com, but I had to use the desktop version of the site.
To add a folder from outlook, gmail, or other web based solution, navigate to the website. Then add the folder you want through the web interface. It will sync through your phone on its next sync, or manual request for sync.