How I can worklog into multiple issues which are already present in JIRA?
Click "log work".
I suspect there's more to this question?
I have to import the worklog through CSV in multiple issues.
You'll need a column to tell JIRA which issues to update. Then have a look at the worklog section in https://confluence.atlassian.com/display/JIRA/Importing+Data+from+CSV
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