Hi Chris,
Since I'm about 40 miles north of you in Hockley, I thought I'd put in my two cents: I have a big, gorgeous in-ground spa connected to my pool but if I had it to do over again, I'd go with a good quality above-ground, self-contained unit. My inground spa is so expensive to heat that I turned off the gas valve and put a lock on it to keep my teenagers from using it...I figured it took about $25 worth of gas to bring it up to temp in warm weather and as much as $40 in the winter. Plus, even with the largest heater available, it takes hours of foresight before you can jump in.
A self-contained electric powered spa will get a lot more use. It has a lid so it stays hot all the time and since the ground is not sucking all the heat away, it only costs about $35 per month to operate.
Lastly, you will find it difficult to find a pool company that is eager to append a spa to an existing pool. They will not want to be on the hook for any structural problems you may have in the future. If you find someone, be sure and check them out thoroughly. In addition to the BBB, you can visit the county clerk's website and look up their DBA filing to see who owns the company and then you can check with the secretary of state website to see if they are in good standing. Try mightily to get a reference from a recent customer. There are a lot of builders, including some with long positive reputations, that are in poor financial condition right now and you don't want your deposit to be used to finish someone else's pool!