As far as I can tell Plecs does not seem to support "csvwrite", "dlmwrite" or "xlswrite" at this time, "save" however does work. A command as below will save your data without headers in text format to a file. If you open the generated file in a text editor and find/replace all space (" ") characters with commas (",") excel will correctly load the file as csv.
data = [ 1:3; 4:6; 7:9 ];
save("-ascii", "data.csv", "data");
There may be a better way but this seems somewhat workable.