I want to develop my own module but still do not understand how it can be installed in TreoPIM. Is it possible? Is there any instruction?


Yes, of course. You can develop your own modules. To install go to http://your-project/#Admin/extensions . The system is based on EspoCRM, it supports any modules developed for EspoCRM, so the development instruction is the same. https://www.espocrm.com/documentation/development/extension-packages/