The Digital I/O pins only support SPI, I2C, static (software-timed) DIO, and exports of the MSO Trigger or FGEN Start signals. There isn't any PWM/counter functionality.
There isn't a way to generate a real PWM signal on the DIO pins, nor can you give a digital waveform to Dig Write. Going from the FGEN might be your best option here.
Another possible alternative might be to (ab)use the SPI mastering functionality. The following VI will generate a pulse-width modulated signal on dig/1 (MOSI). It will also generate signals on dig/0 (CLK) and dig/3 (CS), which may not be desired, and you'll have "gaps" between every call to SPI Write Read.