How to generate HF AC from a DC source?
I think the easiest way to do this is to simply buy an oscillator from CTS, ECS, NDK or any of the other vendors available through the catalog distributors. You could get an oscillator that puts out a fixed 10.000000 MHz square wave at CMOS levels, or, for tunability you could use either a "programmable oscillator" or a voltage controlled oscillator (VCO). With the VCO you could adjust the frequency in real-time using a PWM output from your microcontroller. In either case you will pay a couple of dollars extra for adjustability.

You could also save a few pennies by building your own oscillator, but buying off-the-shelf will get the job done quicker and not require as much testing and debugging. 

Now, how to connect it in to your circuit. As you suggested, you could build an op-amp circuit to scale and offset the oscillator output to a 2-to-4 V range and use that to control a high-side or low-side switch on the LED power circuit.

But more simple would probably be to just AND the oscillator output with your existing GPIO signal, then use the new modulated control signal to drive your existing switch circuit. This will ensure a fast response when the control signal turns on and off.

Here's an idea of how this would look:



The "in" signal is your existing GPIO control signal, and the transistor/resistor combination represents your existing LED drive buffer circuit.