Connecting USB Devices into Computer via Bluetooth?
If the Bluetooth hardware in your PC has Bluetooth Hard Copy Cable Replacement Server capabilities, you can use a Bluetooth USB hub. With the original dongles for the keyboards and mice plugged into the hub, it should be able to pair up with the PC's Bluetooth adapter and transmit all of that data to the PC but you may experience some lag.
There are also 2.4 GHz USB hubs available but they are, in my experience, less reliable than Bluetooth at handling that much data. Check your Device Manager to see if you have the right hardware and drivers:
There are a couple use cases where people were able to convert a USB or PS/2 keyboard to bluetooth and connect to a computer with bluetooth:
https://lifehacker.com/convert-any-usb-keyboard-to-bluetooth-with-a-diy-adapte-1786324129 [includes DIY video]
https://www.youtube.com/watch?v=SRSyqky-WWk
There were also a couple for sale on amazon:
https://www.amazon.com/gp/product/B00T5YXNL0
https://www.amazon.com/dp/B00PI7MDK0/
There is also another situation where you could connect the usb device[s] to a wireless router, and then access the devices via WIFI. An answer here on a different question may wet your appetite https://superuser.com/a/1167644/140631
I was looking for a usb2BT solution myself and then I found myself here. So I figured I'd pass along what I gathered. As of this writing, it doesn't seem like it is worth the expense....or rather it is less headache to just buy a comparable device with Bluetooth >= v4.0 already built-in. But it makes for an interesting DIY project if you have time.