As noted elsewhere there is a certain amount of space inside the S450. In September 2022 I heard from Marius who decided to take a page from HP's design book and use this to install some hidden USB ports. (See the HP t5730 for an example of HP's approach). The ports are 'hidden' to the extent that you need to take the case apart to get at them. These are obviously targeted at fit-and-forget items such as USB dongles or semi-permanent storage expansion.
Adding this feature is quite easy to do as the S450 motherboard carries the usual 2x5 strip line connector that exists on most motherboards for connecting to two USB ports on the front panel. You can also find 'add ons' that use this connector and provide for two additional ports on the PCs back panel. So, as far as the extra hardware is concerned, you can either track down something on eBay (or similar) or scavenge a part from the PC you retired ten years ago and still have cluttering up the garage.
The only problem you have to solve is how to mount your chosen USB sockets inside the chassis.
Marius used a front panel Audio/USB combo stolen from a HP desktop. To fit it he used two identical metal slabs, two screws and one hexagonal spacer.
This is a generic rear panel connector that can be found on eBay. Rather than trying to use it as-is you can remove the two sockets and make your own bracket to take them.
This is Marius's ex HP connector mounted to the metal cross-member in the S450.
Some more detail of the mounting. You can see the hex spacer that was needed to utilise the existing bracket on the HP hardware.