I have a startup, active for 2 years.
I found a guy and it looks like the best way is to get him a J1 visa.
how can my company sponsor a J1 visa for him ?
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You need to discuss it with an immigration lawyer. J-1 is an exchange visa, usually used by employers for interns, post-docs and visiting researchers/professors. Some graduate students get it too. It is not an equivalent of L1 or H1, and there's no conversion to H1b (there's no such thing called "convert to/transfer H1B" in general. You apply from scratch). You have to have a well-defined program for the exchange visitors, its not just a regular employee.
When you discuss the issue with an immigration attorney, ask about H3 visa as an alternative. This is also a trainee/exchange visa, but is more geared towards employers, less towards educational institutions/internships.
Bottom line is that if you're looking for a foreign worker, there's nothing much you can do but to deal with the H1B visa.