Ok, this may be the dumbest question in Bonsai world. but can't help it The picture shows the plant I bought some times back and as you see the root system doesn't look good. There is one large root curved and few other very very small roots. I was wondering whether I can attach an unwanted branch to the trunk to act as a root?
what are the other options I have to get a good nebari?

I would use air layering to create a better rootbase. Grafting roots could be an alternative, but with what you have there.. Nah..

Those are hardy trees so grafting may work. But it may not. Have you practiced this in the past? The more precise you are lining up the cambium the better the odds. Do you have a crafting background? Any fine motor experience will help. BUT...
It looks like your trunk has some reverse tapering at the bottom there. It should only get smaller from the ground up, generally. I would bury that part of the trunk (start at the top of the tree and work your way down. when the trunk stops getting consistently larger, that is where I would make the surface of the soil. Air layering would definitely work but burying it deeper instead could potentially give you more options in the future. Do not worry about a shallow pot for now and make the widest point of the trunk your ground level. Do not prune anything off. These roots will grow fast under organic soil and even better underground in the garden. Even after a season, or especially a year or two, all of those branches/roots under the soil will look completely different. Perhaps luck will strike and beauty will transpire. Everything under the surface will bloat and grow and then your options will improve for the next re-potting.
Or, get multiple of the same tree and try different things with each. This is one of the best ways to learn. They say the greatest bonsai master once killed 1,000 trees while learning as a student.