The .sbd directories hold no mail themselves, they are how Thunderbird represents folder nesting on disk. Understanding the pairing makes the whole profile readable.

Every Thunderbird mail folder is an extensionless mbox file, and when that folder has subfolders, a directory named after it with .sbd appears alongside. So a Work folder with a Clients subfolder sits on disk as a Work file, a Work.sbd directory and inside it a Clients file, with Clients.sbd appearing in turn if the nesting goes deeper. The mail always lives in the extensionless files, the .sbd chain just encodes the tree.

For Outlook this means you do not migrate .sbd files, you convert the whole directory structure. An MBOX to PST converter that understands the layout, SysTools MBOX Converter among them, takes the profile's Mail or ImapMail directory as input, walks the .sbd chains and rebuilds the identical nested folder tree inside the PST.

The mistake to avoid is cherry picking single files out of deep nesting: an mbox file pulled from three levels of .sbd converts fine but arrives as a flat folder with no ancestry, and reassembling a big tree by hand inside Outlook afterwards is miserable. Feed the converter the top of the tree and let it keep the structure.