I understand that I need a telephoto lens, combined with a low fstop to separate the subject in a portrait from the background, which will be out of focus. I'm confused if I add a macro/2x telephoto adapter to this lens will that mean the fstop becomes lower or higher? Will it help me to achieve this objective to buy this lens adapter?
A macro adapter will only allow you to do closeup/macro shots. You'll lose the ability to focus at a normal distance for taking portraits.
Using a teleconverter wouldn't help either as people normally use f/1.4–f/2.8 apertures to separate the subject from the background, not f/11. There's also a big chance that a teleconverter won't even fit mechanically on a 18-55mm lens (the glass in the teleconverter could collide with the glass at the rear side of the lens).
Use the lens you own i.e. the 18 - 55 mm zoomed out to the maximum zoom. In other words, shoot your portrait at 55mm. Use the widest opening available to force a shallow depth of field. Have the subject hold their hand about 8 inches in front of their face. Use this hand target to focus upon. You do this by pressing the shutter button half way down. The camera will focus. Now hold this shutter button position as the subject repositions his/her hand. Complete the composition/pose and shoot. With the focus point now ahead of the subject, the background will go out of focus. Experiment with the hand before the face distance to get the effect you desire. No need to spend money if you can accomplish with existing stuff.
No.. in fact you will get somewhat horrible results.
The lens you'll end up with is a 32-110mm f:7-f11 (ish) lens because not only does the focal length multiply by two, but you are also reducing the maximum aperture by two stops.
The depth of field of 55mm @ f5.6 is exactly the same as 110mm at f8, but the big problem is that at f8 maximum aperture your camera is likely to struggle with autofocus and the viewfinder will become pretty dim.
Not exactly sure what you are trying to achieve but if it's portraiture then on you crop sensor camera you could maybe buy a 50mm f1.8. That's going to be a whole new world of background blur for you.