Yes, there is a classic little circuit I used to use, the exactness is lost in the mists of time. Basically you need an XOR gate to combine the HSYNC and VSYNC to one signal.

Then using a couple of digital transistors and a few resistors you need to create a 1v peak to peak signal such that...

The video signal pulls the signal from 1v down to 0.3v ( white to black)

The sync signal pulls the signal from 0.3v to 0v ( blacker than black )

It's easier if you have the transistors combine the signals at about 3v peak to peak then you a couple of resistors to step this down to the 1v peak to peak signal, remembering the 75ohm standard impedance for composite video equipment.