No, unfortunately maintaining installations of separate PHP versions costs more money, and is just simply infeasible on free hosting. It’s also unsecure unless a solution like Hardened PHP is implemented, but that also isn’t feasible for free hosting. If you upgrade to Premium Hosting however, you’ll have access to a PHP version selector, which uses Hardened PHP. Do note however that if a CMS is old enough to be built on an outdated/obsolete version of PHP, than it likely has security vulnerabilities of its own. Updating your software to something more recent when you can is better in the long run.