Yes, just copy and paste it in Explorer into the new project folder. Reference it inside RH in that project.
I think what the poster wants is to maintain just one CSS file. To do that you would need to put it in Resource Manager and then link to it from there. Rh will create a copy in the project but whenever you make changes in a project (or to the original), Rh should prompt you that the copies are out of sync and allow you to correct that.
Note that other project outputs will only reflect the changes after regenerating the output files.
See www.grainge.org for RoboHelp and Authoring tips
I usually import a topic with the .css and it comes along for the ride.