Can I use (German) Marzipan instead of almond paste?

The correct answer to your question depends upon where you are. In the UK and Germany, and I believe the rest of Europe, almond paste and marzipan are the same. In the US marzipan contains more sugar than almond paste, so you could use rolled-out marzipan on the outside of a cake with no real problem, but for mixing into a cake, you would want to use almond paste. Nigella Lawson suggests mixing an equal amount of fine sugar and finely ground almonds with enough egg white to bind.

I do a lot of baking in Germany and use marzipan--I've never seen almond paste there. Be sure to read the ingredient list--manufacturers put all sorts of cheap sweeteners in marzipan. Some use persipan, which is the ground up 'almond' which comes from almond pits. This is the mark of cheap marzipan and is best to avoid.

if you don't have almond paste where you live, it is very easy to make- almonds, egg whites, sugar, almond extract. Google a recipe.

I would email/ask Sarah Jampel directly...esp as marzipan usually has more sugar than almond paste. Thus you might not want to replace the almond paste 1:1. Or, if you did, you might want to seriously reduce the sugar. That might affect not only sweetness but also moisture and texture of the finished cake.