Is it possible to use the microUSB port as we do with PC USB port with the help of any converter (i.e. microUSB to USB) and connect different devices with it.
it might seem funny to most of the people here bt i need the confirmation.

The N900 (Assuming you mean this) does not support USB OTG and so therefore cannot host other USB devices. Otherwise you can use the convertor and connect it to a host device as per normal.

For the N810 then you need a gender changer and then you can use it to run very low powered devices such as memory sticks (without LED usually) or use a cable to 'inject' power onto the power lines.

More information in other threads, try 'USB' or 'USB OTG' in the search box.

IS USB OTG a software or a hardware.
Can N900 be upgarded to have that in the future?

There's a lot of discussion in this thread; consensus seems to be that it's not fixable.

USB OTG is USB 'On The Go' and is basically referring to the ability of a device to host other USB devices. This means you can add a mass storage device. The N900 does not appear to support this in the hardware itself (Pin wiring etceteras). As the software merely enables and utilises what the hardware supplies I would guess that this is a dead end.

There are Bluetooth USB hubs mentioned though so a workaround might be had depending on your use case. Best of luck!

The only bluetooth usb hub I saw were hubs you plug in the USB port of a computer, with an embedded bluetooth dongle, to add bluetooth through USB, not USB through bluetooth.

Do you have a link to a USB through bluetooth device ?