Currently, it is not possible to clone a dashboard and move all of the cloned reports to another, already existing dataset. The only options currently are to:

1) Clone the dataset too, essentially creating a brand new dataset.

2) Clone each individual report, one at a time, to the dataset you want, then recreate the dashboard from scratch.

I'd like to see the option to clone a dashboard and move the cloned reports to an existing dataset in bulk.