Is it possible to measure frequency upto 500Mhz with 50Mhz oscilloscope?
Not really. 500MHz is too far above the nominal bandwidth for any aliasing frequencies to get through to the digitiser within it.

You can use your scope as a 'back end' for two different types of front end. 

The first is a heterodyne downconverting mixer. You would need a local oscillator, close to 500MHz, and a mixer. The scope would measure the IF, the difference between the LO and the input. It would show you the amplitude of the fundamental of the input waveform as well, sufficiently well to measure it.

The second is a digital divider. Prescalers are fairly inexpensive and obtainable, a /16 would get 500MHz down to 31MHz. It would tell you nothing about the amplitude, except whether it was enough to clock the divider or not.

A lower cost alternative to the simple LO and mixer would be a harmonic mixer. This would allow for a lower frequency LO, but there's more potential for unwanted products to get through to the IF. 

Because all of these methods throw away or confuse some information about the input signal, you need to be more careful about interpretting what you see in the IF. Other input signal frequencies will confuse the output, and differently depending on whether they're harmonically or non-harmonically related, and bigger or smaller than the wanted signal. A bandpass filter at 500MHz could be a useful investment. 

That last paragraph seems quite vague. It's like this. If you know what I'm talking about, then you don't need any more detail. If you don't, then there's no way you can be brought up to speed on what to look out for in the space of an answer here. Look up superheterodyne, intermodulation, and 'how a spectrum analyser works'.