Yes, you can do that. But I would suggest you to it via marketplace or install it first on a Dev environment.
Marketplace already contains lot of plugins, so what's your use-case of creating a custom plugin as maintaining custom plugins requires effort and also you can run into issues while migrating JIRA to newer version thus it's best to download the plugins via UPM which gets it from marketplace.
But still if you want to go ahead with your custom add on, please read here
https://confluence.atlassian.com/upm/installing-add-ons-273875715.html
Alternatively, you can install add-ons by file upload. The file upload procedure installs an add-on as a JAR or an OBR (OSGi Bundle Repository) file. The archive file should contain the plugin code and resource files. File upload enables you to install custom add-ons or any add-on not available on Atlassian Marketplace. You can also use it to install add-ons for applications operating behind a firewall and disconnected from the Internet.
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