Yes, you can do that.
However, before you start to define the Part Family spreadsheet you have to first assign a name to the Solid Body which makes up your part model. That can be done by selecting the Body, pressing MB3 and selecting 'Properties', selecting the 'General' tab, entering some meaningful name and hitting OK.
Now when you go to create the Part Family and the initial dialog comes up, under 'Available Columns', select 'Density' and you will see the name which you just assigned to the Solid Body. Select it and then push the 'Add Column' button. Now when you open the Spreadsheet (by hitting the 'Create/Edit' button) there will be a spreadsheet column with a title that is the same as the name of the Solid Body and it will contain a value for the current Mass Density set in the part file. Now if you're working in Imperial Units (inches) this value will be in 'Pounds per Cubic Inch' and if you're working in Metric (mm) the value will be in 'Kilograms per Cubic Meter'. Now just assign the appropriate Mass Density for each family member, and you should be good to go.
John R. Baker, P.E.
Product Design Solutions
Siemens PLM Software Inc.
To an Engineer, the glass is twice as big as it needs to be.