You can. Liquid in, powder in, cap on and shake. Personally I like premixing formula but I've done it on the go too.
What PP said, though I'd recommend shaking side to side instead of up and down (if that makes sense) - otherwise I find that formula gets stuck in the nipple tip and clogs the opening.
W/DD # 1 we never did pre-mixed, though. We would just mix as needed at home and when we went out we would bring a small canister of formula, a few drop-in sleeves, a couple of nipples and then re-use the same bottle, ring and lid. W/DD # 2 we'll be switching to pre-mixed for at home (she likes to cluster feed, whereas DD # 1 never did, and it's easier that way) but we'll still follow the same process as before when we're out. GL
Edited to add: the drop-in liners are also recyclable, which is a bonus!
Re: Playtex drop ins