Yes you can, and no it doesn't have to be agreed on to be capable of doing it. Being agreed on just makes it easier. But then again, if the other person is expecting to dream of you, than he/she could make themselves do so, no influencement involved. That's merely psychological. It demonstrates a true dreamwalker who can appear in someones dreams or make someone dream something without their knowledge.
When I was dating my husband, everynight before I went to sleep, I would kiss a rose he gave me focusing on them being his lips as he slept. Then I would lay down in bed and whisper good night to him, feeling myself there with him. I did this for a few weeks until one day, on the phone, he told me something weird kept happening to him. Nearly every night just as he fell asleep he would smell my perfume and feel me kiss him. Sometimes he even heard my voice. This dream would wake him because it felt so real, and he would expect me to be there in front of him, but found no one in the room. I laughed and told him that is because every night I do kiss him, then I explained to him what I did. He began looking forward to those dreams and the knowledge that it was not the power of suggestion (as it could be with "agreed" dreamwalking) made it far more special.
How is a hard question to answer, at least for me. I've always had a natural way with dreams. I used to lucid dream and experience OBEs throughout my entire childhood. Very rarely did I experience a dream that wasn't lucid. Even now many of my dreams are lucid but remembering my dreams is less frequent. I like it better that way actually.
I rarely give dreams, but when I do, I play them out in my mind, focusing on them as well as the person I wish to interact with. And fall asleep while doing so. But I have always been able to "start" a dream while awake and "finish" it while sleeping. I'll repeat the same night after night at the same time until the person shows some form of indication that the dream went through. I like giving dreams to people I can't be there for due to distance or just to show them I miss them if we can't speak (such as those who are deployed in the military). So I would guess from my own experience, that learning to dream lucidly and even go so far as to control your own dreams would be the first step to dreamwalking. Once you can master that, its just a matter of focusing on the other dreamer and making that mental connection.
Mind you, I've never read a book on this, nor have been taught it. I never had to be because it came completely naturally to me. But I felt I would give you my best possible answer based off of my own dreamwalking.
"All that we see or seem is but a dream within a dream" E.A. Poe
Realize that night's dreams are no different than daydreams, between waking and sleeping the only difference is sleeping lacks the limits of the physical.