I might be clueless, but are you able to use photoshop brushes?
Okay, I've found a way to use those brush looks, even if they're not compatible in your drawing program.
I use Paint Tool SAI. Photoshop brushes are not compatible and I don't know how to convert them to my drawing program...soooooooo...check this out.
I have Adobe Creative Cloud (Photoshop). I can either export my SAI comic page to photoshop to preserve layers...or save the comic page as a png or jpg image file.
In Photoshop, I can load the brushes then use them in another layer to accent, add texture, or whatever!
Here's a sample page with the grunge and texture brushes!
This is my comic, The Watchman:
These are the brushes used from here:
For a license that enables you to use the brushes without credit is here:
http://www.obsidiandawn.com/grunge-n-swirls-photoshop-gimp-brushes
Hope I've helped.