chasgrav has explained the way to do this with the standard plectrum. I might add that you can do that without changing the grip on the pick at all with just a bit of practice. Not good to change the grip, keep it the same for both methods, just practice using the pick on the lower two strings (for most finger styles) and using the Middle and Ring fingers to pluck the higher strings when needed. With a little more practice, you can even get the pinky finger involved for a few things, like plucking or rolling three strings.

Some prefer moving to a Thumbpick. These have a band that keeps the pick hooked around the tumb. You can also place your index finger next to the Thumbpick for strumming and single note picking much the same as done with the standard pick. Tommy Emmanuel explains how to do this in his courses and videos, you may be able to pull one or two up for free on YouTube by typing his name into the search block there. Players who use the Thumpick in this fashion can play extremely fast single note lines as well as Atkins style picking, "Dead Thumb" Tele-style picking, even standard Classical TIMA patterns because the pick is attached to the thumb.

TIMA - Thumb, Index, Middle, Anular, or the three fingers. Derived from Latin names, which are different but start with the same Letters. You may find T, I, M and A designations on some Classical guitar sheets, telling us which fingers are used on which note of a pattern at the start of the thing, followed by the simile designator, which means to keep on playing the rest in similar fashion.


--Mac