H1B Visa Transfer while in India

[tipintony]

I got my H1B stamped in Jan 2013 from a company A, while in India.

Stamped H1B is valid till 2015, I have not traveled to US yet.

Company B from US is interested in transferring my H1B in premium processing.

I have cleared Company B's Clients interview, hence there is no issue of required client letter for H1B transfer.

Queries:

1. Can Company B file for cap exempt H-1B petition for me based on my existing H-1B petition approval / h1b stamped visa?

2. Is it possible/legal to enter US on Company A h1b stamped visa & company B valid H1B petition?

3. Don't I need to go for stamping again for Company B?