Proprietary, not PCI, but expansion slots, yes. On HPs it was called the MIO slot and later the EIO slot. If your printer came with a MIO card that only did serial and parallel, you could swap it for one that added networking.
And if it stops working, try re-soldering it in an oven before throwing away. Some infamous batches had bad soldering problems that can be fixed. See i.e. [0]
