There is street parking immediately adjacent to the Aloft, but the City of Milwaukee generally prohibits overnight street parking. However, you can apply for a Night Parking Permission that does allow overnight parking (free) for certain streets until the paid parking starts again in the morning. It is really just a matter of logging into http://mpw.milwaukee.gov/services/np_permission, and entering the block address and vehicle information, but it has to be done before 1:00AM.

I haven't stayed there for about a year or so, but I believe that Night Parking Permissions were available for most of the adjacent streets though YMMV. At least some of the parking at the property is in the outside lot which isn't supervised in the evening, parking immediately adjacent wouldn't seem to carry much additional risk if that is a concern.