You can either add the sim to your current household. Go to edit town, select the sim from the bin and move it to your current sims's household icon. Click on the icon and you will be asked if you want to add the sim to that household. You will then have control of both sims.
Alternatively. Go to edit town. Select the sim from the bin and then place it in any of the cheep empty houses in your town, or add it to another household. You will not be able to control the sim. (You can only control one household at a time), Return to your game and have your sim visit the sim you just placed in the town to start the relationship.
You will have to build the relationship through the sim interactions and phonecalls, ask out on date etc. I would suggest you do this to the point of becoming boyfriend/girlfriend fairly quickly or you could find that story progression pairs you chosen sim up with someone else. Switch story progression off can lead to depopulated towns and other problems, so I would not recommend doing this for very long.
You can switch back and forth between households at any time by going to edit town, but, be aware that story progression will continue (unless you switch it off through options) in the house(s) which you do nor control. You could return to a household after a few days and find you have a new housemate, pets, missing items in personal inventory, any manner of things you may well not want to happen.
