Can I add a matching band later if I didn't buy a set?

Yes. Sets are mostly a retail convenience, and I'd rather make you a band later than sell you a set upfront. What you probably want is a band that sits right, wears right, and doesn't fight the engagement ring you already own. That's very doable.

Begin with the profile of the engagement ring. A straight-shank solitaire will accept nearly any straight band, 2mm to 3mm wide, no problem, but the band may not sit perfectly flush if the head sits low, and a small gap is normal and sometimes more comfortable than two rings locked tight. If the gap annoys you, a curved or notched band can tuck under the basket. That's a custom job, usually $400-900 depending on how much contouring the wax needs.

Things get harder with twisted shanks, cathedral shoulders, pavé that wraps the side of the shank, or a very low basket. Those profiles almost always need a shadow band or a CAD-matched contour. I'll scan or photograph the ring, model the band to the exact curvature of the basket, and cast it. Six to eight weeks, sometimes longer if the caster is backed up. You don't need the original designer for this. Any decent bench jeweler or CAD shop can do it, provided they have the ring in hand. Photographs alone are not enough for a true contour fit.

Metal matching is real even when the color looks the same at a glance. 18k yellow from one caster can run warmer or cooler than 18k yellow from another, and the difference shows after a year of wear. If you bought the engagement ring from a shop, ask who cast it. If you're coming to me cold, I'll want to know the karat, the alloy color, and ideally the manufacturer. White gold is a little simpler because it gets rhodium plated anyway, but 18k palladium white and 18k nickel white do not look identical under the plating, especially at the edges where the two rings meet. Platinum is trickier. A new platinum band next to a worn platinum ring will look slightly grayer until the patina catches up. That evens out in a few months.

Last March, a client named Nicole brought in an estate-sale engagement ring with a low basket and the original hand engraving still crisp on the side. She wanted a diamond band. I told her a straight band would either sit proud of the ring or cover the engraving entirely. She didn't believe me until we held a wax model against it. We ended up making a plain 2.8mm shadow band with a small step where the basket sits, no diamonds. She was quietly unhappy for about a week. Then she wore it to work and called to say it was the most comfortable pair she'd owned. The band cost around $680 all in. She could have bought any stock band from a case for $300, and it would have looked wrong.

You can also do nothing. Wear the engagement ring alone. Add the band at a five-year or ten-year anniversary. By then you'll know what you actually want, and you'll have the budget to do it properly. Don't solder them together unless you're sure. I tell clients to wear them loose for at least six months. Soldering is reversible, but it changes the feel and makes resizing the engagement ring more annoying.

So yes: you can add a band later, and usually it's the better move. Just bring the ring in. Not a photo. The ring tells us more than any picture will.