Your text may be a Swiss, Liechtenstein or Austrian text where this expression "auf dem Zirkularwege" is widely used.

What your sentence means is this:

Resolutions may also be adopted in writing by circular vote.

This doesn't mean the resolution is circulated to all members. What it means is that the voting members of the Foundation Council receive a voting form (which is in writing of course) whereby they may exercise their voting rights. Usually resolutions may only be adopted at duly convened Council meetings. A lot of "Statuten" (these are usually translated "regulations" or "articles of association"), however, do allow resolutions to be taken by circular vote (i.e. members do not record their votes at a meeting but complete a voting form).

Please note that "auf dem Zirkularweg" is nothing to do with "by proxy"! If you say "Resolutions may also be adopted by proxy", this would mean something completely different! There are no substitute voters involved. It simply means that the Council members do not meet to cast their votes.

Whether we like it or not, the standard translation for this has become "by circular vote". This is the term usually used in English translations and the expression your (Swiss etc.) customer will most likely expect to appear in his/her "Stiftungsstatuten" (which would be "Stiftungssatzung" in Austria/Germany).