Copying from https://github.com/citusdata/cstore_fdw#usage

  To load or append data into a cstore table, you have two options:
  
  You can use the COPY command to load or append data from a file, a program, or STDIN.
  
  You can use the INSERT INTO cstore_table SELECT ... syntax to load or append data from another table.
  
  Note: We currently don't support updating table using DELETE, and UPDATE commands. We also don't support single row inserts.

So I think you can certainly mutate tables, but the focus is on bulk-inserts, rather than individual append actions.