How to add self-study workload in the ECTS credits?
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.