It depends on how literally you want to stick to the German terms.

Here for "in dem die erste Umwandlung erfolgt" ... you could write "in which earnings are first converted to pension contributions"

a more simple variation would be "in which the first (employee) contributions to the pension scheme are paid" or something like this.

I don't know what you call these things in the US but in the UK we would call this an occupational pension scheme.

German law allows you to "convert" a certain proportion of your gross earnings into pension contributions (Entgeltumwandlung).

Have a look at Section 1a of the Gesetz zur Verbesserung der betrieblichen Altersversorgung