2019 MacPro: Can I put in a second I/O Card?

Yes, slot 8 is special. It's extremely unlikely it will ever be possible to add a second IO card.

If you want more USB-A ports, there are plenty of cheap no-brand options that claim Mac support, or there's a Sonnet Allegro.

If you want more Thunderbolt ports, the only option at the moment is MPX GPUs (other than the base model 580X).

For what purpose, exactly?

Readers also report those cards are not yet available, so discussing alternatives may be "the only game in town" right now.

There are two more ThunderBolt ports on the top of the unit, equally inconvenient for snaking cables out the front or the back.