Would it be possible to eat things from another planet
Short Answer- No, chances are negligible.

Long Answer- Digestion is a chemical process which is mediated by enzymes. Enzymes are highly choosy molecules so that they only perform the work they're made for. In digestion, enzymes like proteases (for breaking down proteins), lipases (for breaking down lipids), amylases (for breaking down starch), DNAses (for breaking down DNA), etc. are involved. For example, amylase is specialized to break down polysaccharides (carbohydrate polymers) into mono/di/tri/oligo-saccharides. See this from wikipedia:


  The α-amylases are calcium metalloenzymes, completely unable to function in the absence of calcium. By acting at random locations along the starch chain, α-amylase breaks down long-chain carbohydrates, ultimately yielding maltotriose and maltose from amylose, or maltose, glucose and "limit dextrin" from amylopectin.


And this image from here:



Lets forget the "completely unable to function in absence of calcium" part as it is out of the scope of this question. But it is necessary to know how specialized enzymes are to answer your question. They are so specialized that amylase can break starch, but not cellulose. See this image:



Source

You must have noticed, in the diagram above, the minute difference between starch, cellulose and glycogen (BTW glycogen is another polysaccharide), but even that difference is enough for enzymes to tear apart one and pass on the other molecule.

The above explanation was necessary to tell why the answer is no. If we eat something, enzymes are needed to break it down. But if enzymes cannot recognize something, then they can simply not work on it. So, if you even want to eat space grass, forget about being able to digest it. Because even if we confirm that life on earth came from space through meteorites billions of years ago, then also during this time period of billions of years both that life of space and this life on earth would have evolved so much that they might not match, even chemically!

Hope this helps. Tell me if you want any more details :)