Unfortunately, space does not exist to manage more gibbon species. Seeing as not even white-handed or white-cheeked reach the criteria of a signature program, and are currently provisional, there's next to no chance of adding a fourth gibbon species to the SSP programs too.
Considering that gibbons are monogamous apes, and most zoos who house gibbons will house a single pair, it takes a lot more holders to create a sustainable population than it would in, say, a penguin species where individual zoos house 20 plus individuals.
These populations already are connected. Javan gibbons are managed globally, so the three US holders are working together with the holders in Europe and Australasia. The white-cheeked gibbon program works as much as possible with other regions, however it isn't always feasible or advisable to move animals overseas on a regular basis, although some cross-region transfers have occurred relatively recently (in both directions). There really isn't a pileated gibbon program in the he largest pileated gibbon holder in the United States is an unaccredited zoo that no reputable organization in their right mind would want to associate with.
