Unfortunately no. Mailspring does not support importing emails from other desktop applications (like Thunderbird or Evolution). But that's generally fine! When you connect your email account to Mailspring, it automatically fetches and indexes the messages available on the server and indexes them for search locally on your computer. The process can take a few minutes or several hours (depending on the speed of your network and the amount of mail you have) but only needs to be performed once.
Many older clients like Thunderbird provide an "offline mailbox" using the mbox file format, allowing you to delete messages from your email provider / IMAP server and continue to view them on your computer. Mailspring does not support this use case—if you delete mail from your email account it will be deleted from Mailspring's cache, and the cache is routinely pruned to prevent Mailspring from using too much disk space. Over the years, email archival has become less important—many mail providers that used to provide 10MB or 100MB of storage now provide 10GB or more—and we've decided to focus on building a client that balances local storage with network retrieval of your mail.