Can a person influence themselves to be more along the lines of another sign, even if they don't have it in the chart? For example I might not have anything in Aries, but I have a first house. Can I focus really hard on 1st house sign affairs or something to draw more of that into my life.
Apologies if that's confusing just a thought.
In a somewhat related question, can you help yourself interact more easily with this or that planetary energy?
Yes, you can! This is the value of interacting with people who have planets opposite yours. You can learn from them.
For example, my husband has Aries opposing my Libra. I have really learned to be more decisive and act rather than endlessly waffling.
Great question!
It is also possible to become "more Aries" by weaving together Mars and Saturn where ever they might be in the natal chart.
Saturn describes what is necessary - what we must do. Mars is the motive energy or force which gives effect to fulfill what is needful and necessary. Put the two together and they enhance and support each other.
I don't know about becoming more Aries, but I DO know that Aries everywhere will be pleased at your effort at being more like them.
And wonder why everyone else isn't doing the same 
@user "And wonder why everyone else isn't doing the same"
That made me laugh out loud. An Aries ex of mine would have been deadly serious when relating to that statement and would be wondering the exact same thing! Best chuckle I've had all day. Thank you, I really appreciate a good wit 
I'm wondering how hard someone needs to push to bring in these energies? Maybe removing influences of dominant planets in the chart in real life would help as well.