Yes, you can do it. It does not matter if you call a Jira rest function or any other program Rest function. It is all the same.
You can find an example here
Rest endpoint in scriptrunner let you develop your own rest endpoint in Jira. You can call it from an external application. In other words the external application can call your Rest Api developed in Scriptrunner. But as far as I understand you want the opposite. You want to call external application from Jira.
As Alexey mentioned you'll want to use that example of HttpBuilder to call out to external REST services. It will work from anywhere within ScriptRunner.
You can also use RESTClient to do the same.
Script REST endpoints are used so other people can call JIRA or if you have a front-end component that needs to call the server side of JIRA to produce some effect. For example update an issue.
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