Yes there is, it's called a computer :-)
Like you said, you have an USB MIDI controller. This type of device is designed to be connected to a USB host, i.e. (in general) a computer and used with a Digital Audio Workstation (DAW) software.
However you can install a MIDI (standard) port in your computer connecting also to a USB port. The Roland/Edirol UM-1 is a commonly used device for this, but there are alternatives from many vendors.
Then you can use a DAW software to route the MIDI messages coming from your controller, through the output port of the USB MIDI port, out to your synthesizer.
If you don't have a DAW software you can use a simpler MIDI host software, there are a few freely available out there. I suggest for example Hermann Seib's VTSHost. As the name implies it's specific purpose his to allow the usage of VST plugins outside the context of a fully fledged DAW. But it's perfectly capable to manage several input and output MIDI devices. Don't let the disclaimers and not so professional look of the site fool you, I've been using this software for a number of years and it works great.