How to add self-study workload in the ECTS credits? I'm applying to a Master's program in "Mechatronics", in Germany. One of the program requirements is a ECTS credits form for some specific fields.

According to the university I'm applying (Tuhh) the workload indicates the time students typically need to complete all learning activities (such as lectures, seminars, projects, practical work, self-study and examinations) required to achieve the expected learning outcomes.

I'm not sure how to convert my degree credits/hours into ECTS, given that ECTS points do include studying and assignments hours.  

In my university the credits assigned in the transcript of records correspond ONLY TO LECTURES (classroom hours), where 1 credit is equivalent to 15 hours, even though all lectures are accompanied by homeworks, exams and self-study time.

I can convert the credits on my transcript but there is a clear lack of required ECTS credits. This is because only the workload of lectures are included and the self-study hours are not explicitly incorporated in the university transcript which leads me to consult how to proceed to complete the form.

I'd appreciate if someone could enlighten me. Please, let me know if more information is needed in order to calculate the ECTS.
You can estimate it: if your previous programme expected you to work full-time on it (40 hours a week), and let's say you have 3 courses per semester, then each of these courses would be 10 ECTS (assuming equal load). As there is 60 ECTS in one year, i.e. 30 in a semester. 1 ECTS is equal to 28 hours by the way.

But calculating ECTS is hardly something you need to be doing on your own. You should approach the German university about this, chances are they have conversion tables, or at least some precedence on how to convert these figures. Moreover, the ECTS requirements is just a metric of what they actually want to measure: relevant experience. So you can also discuss your personal case with them and provide them with the study guides of the relevant courses you've followed to see if you fullfill their requirements.