The whole point of the brand is to NOT have to build or acquire any hotels. It is simply offering independent hotels a soft brand to keep their own identity while gaining many of the same benefits of being a "name brand." Thus Starwood simply has to negotiate with existing hotels as to how much money they would be giving Starwood in exchange for getting into SPG and the Starwood distribution and marketing network.
For clarity, Design Hotels as it currently stands is NOT a brand and was not a brand before Starwood acquired the company. It is simply a company that provides ad hoc services with the hotels being promoted and rooms sold through the designhotels.com website. They can also help with PR and yield management, etc but it is entirely up to the hotel as to which services they choose to pay for and which they simply do in house.
Starwood has already announced that they would like to work with at least some of the hotels currently being marketed through designhotels.com and bring them closer into the Starwood fold--but Tribute would not be limited to those hotels. Any place that Starwood wanted to increase their presence which had an independent hotel which fit the new bar for joining Tribute would be a potential candidate for the new brand.
Of course in the future, some company could also build an independent hotel and negotiate with Starwood to join Tribute as well but I doubt that will be the focus for the short term as it is far easier and quicker to ramp up the number of hotels in the brand by negotiating with existing owners--or hotels that are at least close to being opened.