SoapClient.__setCookie() and an array as value?

Hello everbody,
does anyone know if there is a possiblity to give the SoapClient
__setCookie() method an value which is transformed to array values?
I debugged what cookies looks like when you get them via
SoapClient._cookies. If you call something like
SoapClient__setCookie("testkey","testvalue") the result is an array
with the name "testkey" and one element, the given "testvalue". But I
could not find out how I can give more than one value to that array.

Have someone a hint for me?