How do I add individual Staff Time Signatures in Dorico?

I still have Dorico 3.x (the one right before 3.5) and will update soon.

In the situation that let’s say for a String Quartet, I would like Violin 1 to be in 9/8, while the other 3 instruments are in 3/4, How would I do that in Dorico?

See the Operation Manual here.

However, I assume you want one dotted quarter in 9/8 to equal one quarter in 3/4, in which case you will need to use an alternative approach, because at the moment Dorico does not support differing metric modulations between different simultaneous time signatures. You should probably use 3:2 triplets on your “9/8” staff, then hide the tuplet numbers and brackets via Properties.

https://www.steinberg.net/forums/viewtopic.php?f=246&t=151696&p=816877&hilit=Bleibet#p816877