Using CSV files to import custom data into Jira is the most versatile approach since many systems can export their data into CSV. However, as you will have noted already, Jira comes with a number of specialized importers for various systems. These importers often have additional features to help with data import. The Atlassian Marketplace website (https://marketplace.atlassian.com) also has a number of importers created by third parties. If you do not see your system listed in the out-of-the-box importers, make sure you do a search in the marketplace and check whether someone has already created an importer for it.