You can indeed, and this "feature" has been widely advertised by Hasselblad's marketing literature. However, I believe it is the wrong approach. Choosing a 6x6 with the intent of eventually cropping to, say, 2:3 is adding an unnecessary level of complexity to the process. In my case, if I shot a picture with the intent of cropping it later, I would forget what I originally had in mind by the time the negative gets enlarged in the darkroom - but that's just me
Choose a camera for the aspect ratio you want. The great thing about MF is that pretty much every format is available, including 3:4 (6x4.5 or 6x8) or 2:3 (6x9) up to more exotic ones (6x12, 6x17 etc).
As a side note, square format can be learned, even if it is not intuitive at first sight. Composing on a 6x6 ground glass is a different experience than on a cell phone, I'm sure you will get much better results.