When you're ready to go back to DTD, from a park, just take a bus to a resort (Pop is a good one) and transfer to the DTD bus from there. You could also take the bus to SSR and walk but that's going to be a longish walk. You could go to POR, POFQ, or OKW and take their boat back to DTD.
The only park that has had service to DTD was AK. But...that was only later afternoon to park closing. That service has been discontinued as it was only a test at the time.
Disney hasn't felt the need to provide buses to, and from, the parks at DTD. There is little need. If they did provide buses for this purpose, parking at DTD would be truly clogged. People, staying off-site, would park there and take buses to the parks. Parking is already not so wonderful at DTD....allowing park access would merely exacerbate the issue.