This question might be over-answered but I could not find one. Basically I am using RStudio and the keyboard shortcut cmd + shift + c for inserting comments. Is there an other combination to insert directly the roxygen tags #' ? Or a way to modify RStudio to tell it to add the ' when I press cmd + shift + c?
You could use an RStudio addin, you'll need a fairly recent version of RStudio. I've just created an RStudio addin that comments/uncomments using roxygen2 tags, i.e. works just like code commenting, but with #'. The addin is hosted on github.
Just install and attach a convenient keyboard shortcut.
If you are interested in other available addins, see the addinmanager addin.