Hii aspirant..

only those candidates are eligible to apply for improvement if they pass class 12th examination. And for NDA a candidate should have passed class 12th from any stream for Army and he should have passed class 12th with pcm for Navy, Air Force and Indian Naval Academy.

Thus if you clear your improvement exam you are eligible for NDA exam and there is no separate form for improvement students if you have cleared your improvement exam then you are eligible for it.

Hope it helps. 

Best of luck.