Can I switch a single speed mag rim into a six speed?
Going from single speed to multi speed is akin to re-building the whole wheel, since you'll usually need to swap the hub and consequently need to put in new spokes and stuff. 

However, a cursory google search shows several mag wheels for bicycles which take cassettes. I can't vouch for their quality or safety, but they exist. 

A 6 speed "cassette" is almost surely a freewheel (i.e. the mechanism that allows you to coast without pedaling is in the thing that holds the sprockets, not in the hub of the wheel) and not a cassette, and this is a fairly outdated design. You'll likely end up upgrading to a 7/8 speed cassette if you make this switch (which may also involve spreading your frame to get the new wheel in, if possible (i.e. it is a steel frame)).