Is the battery grip for 1000D(BG-E5) a direct fit on 1100D ? I was searching for aftermarket and Canon brand ones but they mention that its suitable for 1000D.
No the Canon BG-E5 is not officially compatible with the 1100D. Canon have not released a battery grip for the 1100D, neither does it appear they are planning to, since the SD card slot is inside the battery compartment you'd have to remove the grip every time you changed memory cards.
This appears to be another "feature" that is deliberately missing from this line in order to separate it from the xxxD line.
There seem to be a few third party grips produced but I'd worry about the quality. However a much better option if you're thinking of upgrading is the T1i (500d), which is compatible with the BG-E5!