Unfortunately this isn’t as easy as it should be at the moment, though it is usually possible! Depending on what format you’re currently using you should be able to convert it to the KeePass file format which is used by Strongbox (and many other open source apps across platforms).
You can find some helpful links below to help you get started. Often a quick Google search along the lines of “Convert
In the long run, conversion from popular formats is on the development roadmap for Strongbox, so hopefully this process will be a whole lot easier soon!
Popular Format Conversions
Converting from LastPass to KeePass
Converting from 1Password to KeePass
Simple Manual CSV Import
You can also construct your own CSV file in the following format which is a quite manual process but you might find it helpful:
There is also a lot of helpful information on the official KeePass site here: