Is it possible to create an additional modules?
If it is possible, how to make it?

For example: i'd like to create Planned Maintenance Module.

Thanks

Yes, it is possible:

1. Add a new "my_module" line in "modules.xml" file

2. Create the folder "my_module" in {app} folder

3. Create the "menu.xml" file inside "{app}/my_module" folder (see the other menu.xml files)

4. Create subfolders for each model where you have and create the files: model.xml, form.xml and list.xml. Also create the standard controller php file for your model.

You can learn by example from the XML files in the other modules. You should also remember to create your MySQL tables needed for your own models.