Is it possible to add a rough sheet surface in order to simulate painting on a rough surface in real life?
The simplest way is to add a rough texture layer. You can use one from the supplied materials or you can File> import >Pattern Image. You have to put this texture layer above all the other layers and then go to Layer property palette and set the "|Effect" to "Texture Combine"... If for some reason that option is not available then set the layer blend mode to "overlay". You can add this layer at any time (including the end) as it does not affect how you paint in just makes the existing image look rough.
There are two ways I know of.
The most obvious is to modify the brush itself to use texture. Check the rough pencil or other textured pens/brushes to see how this can be achieved.
A second way I have used for grunge texture in mono (black and white) layers is to use a screen tone. This provides a gritty inking medium that looks like photocopied art or ink on rough paper. Use a layer set to greyscale then set it to use a screen tone. Modify the screen type to noise and tweak the settings. Then draw on the layer using soft brushes or heavily antialiased pens.
This technique will produce a stippled black and white look reminiscent of certain rough media which has been traditionally photo-reproduced. As a bonus you can use both brush opacity and grey levels to achieve the look you want, and hard (non-antialiased) brushes such as the g-pen can be used on the same layer for sharp lines. You may have to fiddle with the screen settings until you find settings that you like.
Perhaps not exactly what you were looking for, but a nice technique and very useful on occasion for a looser look.
A third way is to combine texture after painting using layer blend modes, but this is effectively how most people do it in photoshop. So check tutorials on layer blending in photoshop - clip studio directly supports most of the methods you will find for that application.
@roar are you talking about your tablet or software?
@RareWill I was thinking of the software :)