recirculation of hot water

I want to install a hot water recirculation pump. I’m not sure how it fits in with the hot water expansion tank on top of the water heater. Do you have any tips?

There are numerous 'kits' that contain (nearly) everything you need. If you don't have a dedicated return line, then you use the cold water line. This will cause the water in that line to get warm (to hot) depending on the settings. It gets purged out when you draw cold. Some put the pump at (near) the WH, and some put it under a sink furthest from the WH (I have one of these installed - a Redy-temp unit). All systems require a check valve and some sort of control valve(s) at the sink(s) to create a cross-over to allow the line to be purged. The unit I have has it all in the box, and takes about 10-minutes to install IF you have a place to plug it in. You can have the thing run on a timer, a proximity sensor, or a momentary button, or continuously. A dedicated return line is often plumbed into the drain line at the bottom of the WH. At a minimum, the system requires a potable water pump (brass or SS, not an iron one) and a checkvalve (and cross-over, if using the cold line). The better retrofit units all have some sort of thermostatically controlled cross-over valve underneath the sink(s) you want to have the hot water sooner.

I have used a Laing 303 system for several years and find it to be excellent. I'm sure other brands are also good, but this is the one I happened on. I have attached the link to the Laing site, You can see there are many models to fit different needs and situations. For me, it was very easy to plumb a return line, so that's what I did. There are models that do not use return lines, but to my way of thinking they are not too appealing.
http://lainginc.itt.com/LG-pump-Circulating-Pumps.asp

A separate return line IS preferable, but in most cases, it is NOT a practical option for economic reasons as a retrofit. The circulation system has NOTHING to do with the expansion tank, since they are two different items to address two different situations. The Laing 303/Grundfos Comfort systems are an expedient means of getting recirculation water, BUT they are not a panacea, since they do have a slight downside when operating, and the "thermostatic valve" WILL fail and have to be replaced periodically. The Laing fits under the "furthest sink" and needs an electrical outlet in the cabinet. The Grundfos attaches to the heater's hot water outlet and the valve goes under the furthest sink, (you can use multiple valves for complex piping systems. Laing also has a version like this).