You can use Java's own conversion to move between HSB and RGB values. For example:

import java.awt.Color;

void setup()
{
  size(400,400);
}

void draw()
{
  float hue = 0.2;
  float sat = 0.5;
  float bri = 0.8;
  background (Color.HSBtoRGB(hue, sat, bri));
}

1. color mycol = Color.HSBtoRGB(hvar,svar,bvar); 

1. color mycol = Color.HSBtoRGB(hvar,svar,bvar,avar);

Yes you can store colour in that way. The alpha value should not be part of the HSB to RGB conversion, but you can add it in easily when you wish to do something with the colour (note also that by default the alpha value will be scaled between 0-255 when used in this way). For example,

import java.awt.Color;

void setup()
{
  size(400,400);
}

void draw()
{
  background (255);
  
  float hue = 0.2;
  float sat = 0.5;
  float bri = 0.8;
  float alf = 128;
  color myColour = Color.HSBtoRGB(hue, sat, bri);
   
  fill(myColour,alf);
  rect (20,20,100,100);
}