Hmmm... I've had a couple of different Sims married to service NPC's - one to the maid, and one to a cop who showed up to catch a burglar! All I had to do was talk to them when they were initially at the house, then call them later and invite them over. My sim then went through enough social actions to become at least "Good Friends", then went through every available Romantic action (always start with Ask If Single, Flirt, Compliment) until I get the chain of options referred to above here - Confess Attraction, First Kiss, Propose Going Steady, Propose Marriage, Private Ceremony. If your Sim has completed the Celebrity skill challenge (Charisma) then you should already have a good initial relationship boost. Combine it with Attractive and you get instant friends when you meet strangers! That is what helped my writer sim Eifrig Wortschmidt woo and marry the lovely Ursula (policewoman) in a single day!
