How to extend Nikon D3000 DSLR zoom?

I own a Nikon DSLR D3000, and I'm Using a Tamron 70 - 300 mm Lens. I'm not satisfied with zoom I'm geting, I Can't go for a Super Tele in my budget. Can anyone suggest Lense Convertor for higher zoon. What is other option for to get high zoom as that of a Bridge Camera.

You may want to switch to sigma lens' they work well with most cameras including yours and are far less expensive then ones like Nikon, Canon etc.

I use them with great results. Please see the below link to their site.

More reach would usually call for a fast telephoto lens and a 1.4x or 2x teleconverter.
With that lens you won't be able to use such a teleconverter unless you are willing to lose autofocus.

Those crazy 800mm (full frame equivalent) focal lengths of bridge cameras hence are not really available for DSLRs unless you intend to invest a lot of money.

You could try cropping the image later in the computer. There's is plenty of quality to spare in the image from a D3000 for a two times crop, or more, instantly turning your 300 into a 600 or 800.