Yeah - two things you’ll want to do is learn a little about caching poses and working with Slots. The best documentation generally has to do with Anim Montages… but maybe this pic will help give you a general idea of how it would work.
A cached pose is kinda like storing the results of everything upstream into a variable. (Its not that simple but its an easy way to think of it). Using your descriptions, lets say the top “Use cached pose” node (feeding into the green blend node) is storing everything you want related to your normal everyday running animation in it. The bottom one has an animation of the guy pulling out a flashlight.
What a slot node does is say ‘Hey, from my left shoulder down thru my hands - play this other flashlight animation’. Meanwhile the rest of the body is still running like normal. The example in the pic has a slot from the waist up but you could set this anywhere in the skeleton including your left arm.