Not sure if I fully understand your questions but based on my limited understanding:
Are you asking how to save an assembly and all of its child parts into STEP files all at once? If so, you can do a Save As Translated and set the "Save as type" to STEP. If you are exporting a STEP file of the .asm file, all of the children that are used in the assembly will be incorporated into a single STEP generated from the .asm file.
If you are asking how to save an assembly and all of its child parts into individual STEP files all at once, then this cannot be done from inside of Solid Edge. Instead you will need to use the custom Batch.exe program to export all the files in a folder to STEP. This Batch program can be found in "C:\Program Files\Solid Edge ST8\Custom\Batch\Bin".
As to the "opensave.exe" program, this is another custom program that will automatically open and save a provided list of Solid Edge files. This utility is used for opening and saving large quantities of Solid Edge files into the current version of Solid Edge. The program can be found under "C:\Program Files\Solid Edge ST8\Custom\OpenSave\bin". A readme file is located at " C:\Program Files\Solid Edge ST8\Custom\OpenSave\readme.txt".
If this does not answer your questions, especially on the STEP question, please provide further clarification of what you are attempting to achieve and I will answer further.