You have not clear what's an USB host and USB "client" interface.
For example, a PC has USB host connections, so it can accept USB client.
USB hosts are not trivial devices, they have capabilities to enumerate, recognize, interface and often give power to the clients.
Pen drives, printers, scanners, webcams, bluetooth dongles and so on are USB "clients" and can be connected only to USB hosts, and not between them.
I am (almost) sure that your board has NOT an USB host interface, but it's only a client.
I suspect that the USB interface present on your board is only a conversion of the existing RS232 (serial) interface.
As already told you the only way to go, in my humble opinion, is to evaluate the feasibility of using a RS232-bluetooth interface.
I saw few of these devices and the are usually pricey (around 100 USD, according a quick google search).
The problem you *may* have with this kind of device is the need to program/initialize it from its RS232 side, and this cannot be done without modifications of your board's firmware. You need to choose a device you can transparently use in you application, converting the wired RS232 interface to a wireless one.
On PC side you can use a common and cheap BT dongle.
Beware of timing issues: if the software you will use has some critical timings, the additional delay inserted by interface may disrupt the protocol.
Regards