I like Dropbox more than Google Drive. Currently when there is an attachment in GMail, there are two buttons - download and add to GDrive. Can I somehow replace or add a Dropbox button?
No, "download and add to GDrive" copies the file between Google servers without downloading it to your computer. That's why it's built in to Gmail.
You can download the file to your Dropbox folder on your computer. (You could save steps by putting a Dropbox-folder shortcut where your "Save" dialog can use it. This varies by your desktop OS.) But you still have to save into your Dropbox folder.
For Gmail to implement "download and add to Dropbox" directly (without going through your local computer), Gmail would need the ability to log in to your Dropbox account.
(You might try it on Android. On Android, both Dropbox and GDrive register as storage services, and you can remain logged in to both of them. So when Gmail or any other app downloads a file, it could let the user pick where to store it.)