Is it possible to chemically treat grass to make it digestible?
The indigestible component of grasses is cellulose, which is polymerized glucose.

Yes, it is possible to hydrolyze the cellulose to glucose, which is of course digestible.  

The hydrolysis can be catalyzed enzymatically with a cellulase, or using acid hydrolysis.  

The process of hydrolyzing cellulose is often discussed in the context of so called cellulosic ethanol, as the glucose is then fermented to ethanol subsequent to hydrolysis.