Technically, you can do anything you like if you choose to write custom coding. So, yes, captcha *can* be installed to the admin login page.

However, I submit that that's the wrong idea.

If you are having unwanted visitors to your admin area, you should be doing more effective things to block access, such as:
a) Rename your admin folder to something obscure ... that only you know.
b) Don't post links to your admin folder anywhere on your site. If you need a quick link, put a bookmark in your browser instead.
c) And if you need a double layer of security beyond that, use .htaccess password protection to set a 2nd password on your renamed admin folder ... which must be entered before even being prompted for the store's admin password.

These ideas are discussed elsewhere on the forum, and some are referenced in the Security Recommendations FAQ article.