Is it ok to ask for H1-B visa during postdoc interview? How?
Generally in an interview before you've received an offer, your overriding priority is making yourself look like an appealing candidate, and so bringing up points that will present a difficulty for your employer may not be so wise.  Trying to negotiate a point like this will at best put your interviewer in an uncomfortable spot (since they are almost certainly in no position to commit on a point like this) and at worse come off as arrogant.  I think it would be reasonable to ask if they can put you in touch with a staff person who can explain how visas are usually dealt with (it is probably not wise to try to look this person up beforehand separately).  

After you have an offer, then you are in a better position to make requests, though honestly, most postdoc positions are pretty much "take-it-or-leave-it."  Visas (as opposed to salary or research funds) are even trickier since they are not usually handled by the department, and have legal constraints that most of the parties involved understand poorly.  You can try, but I wouldn't have a lot of optimism you will get them to do things differently than they would have.