The only way you can convert from Plesk to DirectAdmin is manually.
First: You WILL need to create all your resellers and users on the DirectAdmin server manually.
You'll need to manually set up the domains under each user, manually, including all aliases, redirects, etc. using DirectAdmin controls.
You'll also need to set up all the email virtual addresses, forwards, aliases, (DirectAdmin considers aliases and forwards as one and the same), etc. DirectAdmin has a simple mailing list structure which uses Majordomo. If your accounts and Plesk server use something else for mailing lists you'll need to install that manually.
Don't forget to set up SSL sites as reqired; give each it's own IP# and decide if you want to have all site content in public_html, or if you want insecure (http) content in public_html and secure content (https) in private_html. Set DirectAdmin accordingly.
Once you're sure each account is properly set up, you can consider moving content:
HTML: DirectAdmin stores content to be delivered either in public_html (for both secure and insecure content) or public_html (for insecure content) and private_html (secure content), so move your content as required. I can't discuss permissions because to a great extent that depends on how yu've set up PHP to run. But generally all content for all users should be owned by their username and usually (on CentOS anyway) with the usernme's group.
CGI: If you've got CGI content to be sure to put it in DirectAdmin's cgi-bin directory for each user.
MySql: DirectAdmin uses this naming convention for databases: username_databasename. So you may need tp reme your databases. If you do, you'll likely need to change files in your sites to address databases by their new names. If you can get standard MySQL exports of your databases you may be able to import them using the DirectAdmin control panel; otherwise you may need to use phpmyadmin (from inside the DirectAdmin control panel). You probably won't get far asking here if MySQL databases run into issue son DirectAdmin; if so it's generally a version/feature issue you'll have to work out. Google is your friend.
EMAIL is the hardest it gets. DirectAdmin and Plesk have both been using IMAP for many years, but store and manage email differently. Does Plesk still use qmail? DirectAdmin uses exim. DirectAdmin uses dovecot to manage both imap and pop3 delivery. DirectAdmin does use the standard IMAP file structure, but puts it in different places. By default DirectAdmin uses exim to deliver email to mailboxes but lots of us (including me) switch to using Dovecot, since with dovecot we can integrate sieve into our webmail clients and give the clients email filtering based on content.
You're welcome to try to move email using command line tools, but every time I've tried I've given up. Instead I've used a desktop IMAP client, logged into the old server and the new server, under the new server login created the filke structure manually, and movedfrom the old server to the new one from the desktop IMAP client. Slow and Tedious, but it works and doesn't give me fits. If you want to use the command line to save time you're a braver man than I am; have fun.
Webmail? Does Plesk still use Horde? DirectAdmin offers several webmail clients, but not Horde, so either switch your clients to RoundCube (preferred gecuse it implemtns using sieve to filter better than most) or install Horde yourself. There are threads on these forums which may help.
What did I leave out? Probably lots. And note of course that it's been over ten years since I've last migrated a Plesk client base to DirectAdmin. I take NO responsibility for any mistakes I may have made in these posts, even if they cause you to lose your data. I'm not available to do the work for y ou, and if I were you likely can't afford even one hour at my hourly rate because if y ou could you could certainly afford a month of a CPanel box so you could go the Plesk -> CPanel -> DirectAdmin route.
Good Luck!
Jeff
