You can input time signatures using the time signatures popover, both during note input and by adding them to existing music. You can also input time signatures only on single staves.
Dorico Pro does not automatically add beats to fill bars according to the new time signature at the end of the affected region unless Insert mode is activated.
Procedure
Result
During note input, time signatures are added at the caret position, even if this is in the middle of an existing bar.
When adding time signatures to existing music, they are added at the rhythmic position of the earliest selected item. They appear to the right of barlines, key signatures, and clefs, and to the left of other items, even if this is in the middle of an existing bar. If you selected an existing time signature, the new time signature directly replaces the existing one.
All subsequent bars follow the input time signature, until the next existing time signature or the end of the flow, whichever comes first. Dorico Pro automatically inputs and moves barlines as required so that subsequent music is barred correctly. However, in open meters, you must input barlines and add beats manually.
Any time signatures you input after an interchangeable time signature that are specified in it are hidden automatically until you end the interchangeable time signature.