Hello Robert, this is not currently possible in PDM using one of the list type data card control (Combobox Dropdown, Combobox Droplist, Simple Combobox, Listbox). They all only allow you to select one value at a time.
See the enhancement in the SolidWorks Knowledge below:
It might work using the Checkbox method that you are suggesting, or multiple text fields, otherwise I imagine it could be done through the API perhaps.
Yeah, until the enhancement comes in, I'd try to go with the checkbox route. Checkboxes will put in either a 1 or 0 in the variable field (Word custom property)....but since you will likely have a Word macro anyway to update your Word fields, you could tweak the macro so it will convert the 1's and 0's to what ever value you would like them to be.