You need to be more specific about the application. What do you mean by impedance and impedance to what and what are the voltage and currents in question?
Are you talking about a controllable termination in a high speed bus, variable resistance for voltage controlled gain, or a variable high power load? Any of those things fit the description of 'impedance' (an over-used word). There are ways to solve all of them but they vary widely.
Regarding your comment, the application is basically to lower the impedance of a node, or better saying to inject a current in a node. I mentioned impedance, what I meant was to lower the resistance of a node to inject a current or by injecting a current you are already lowering the resistance in the node. I don't know how to call it.