You can fake this fairly easily. For the notes, use triplets and hide the numbers and brackets.
To make the 6/8 time signatures, press M and type 6/8,4 (to create the 6/8 signature with a pickup bar with four 8th-notes) and press Alt-Enter to create the time signature on only one staff.
Then create a 2/4 time signature at the start of the next bar, and hide it.
This is what the score looks like showing the signposts for the hidden items.
There is a limitation with this - you can’t “fake” a time signature change in the middle of a bar, because Dorico will create a new bar line and there isn’t any way to hide it.