You want to use the File->Export->User Defined menu command in Magpie. You need to do a little configuration of the user defined format the first time you export - see the Appendix in the Moho manual.
When you do export, it doesn't seem to care what file extension you use. You could name your file myfile.abc, myfile.txt, or myfile.dat. Just choose a name that ends in .dat, and you'll be fine.