If you want the conversion to always succeed, probably the best way to convert the string would be to consider "1" as true and anything else as false (as Kevin does). If you wanted the conversion to fail if anything other than "1" or "0" is returned, then the following would suffice (you could put it in a helper method):
if (returnValue == "1")
{
return true;
}
else if (returnValue == "0")
{
return false;
}
else
{
throw new FormatException("The string is not a recognized as a valid boolean value.");
}