I have not had a look at your project but, yes in Dorico you can double the duration and you can also choose any time signature.
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Be aware, if your music contains tuplets, it is possible— but not straightforward:
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I had a look at your project file now.
Yes, you will need to do some cleanup in the first bars. That’s normal if the rhythms in the source files don’t add up under the hood (the file comes from Finale, that could be a source of errors. Either you run a plugin to check for rhythms directly in Finale before exporting, or you fix this in Dorico).
What I normally do is: Select all, Filter Notes and Chords, Filter (deselect) Grace notes, Filter (deselect) Tuplets.
Then in the Property panel uncheck “Start Voice” and “End Voice”. This will make all hidden rests appear and gives you a chance to correct. You will see that some bars don’t add up to 4/4. There are methods to fix this, if you know your way around Dorico a bit.
But it might be better for you, if you can fix this in the Finale file prior exporting.
Once you have done the cleanup, you can actually do the double note values (with Insert Mode on) and change the time signature.
If you could also provide a screenshot or pdf of the first few bars of your original file, we could tell you how to correct the rhythms. Have a look at the beginning, obviously there are more notes than will fit into bar one. Are they supposed to be part of a second bar? Where should the singer place the “but”? Where should the triplet “g a-flat a” of the piano be played?
