Is it too bad to accept job B offer, and one week latter accept job A offer, and "reject" job B?
Depends on how the company will react.
Accepting an offer binds you to that contract. Depending on how the offer is worded, you might have to pay the company some penalties because when you accept a job, a whole bunch of stuff happens on their side: looking for other candidates is stopped, other valid candidates are notified that they will not be getting the job (because it was offered to you), logistic processes are started to bring you on board, legal details need to be handled, etc. If they do this and then you tell them "sorry, I will be going some place else" they will not be happy. They might go after you in one way or another, black list you, your reputation might be affected if words travels around, etc.
There is also the option they will do nothing, just let you go, but most companies will protect themselves in some way from this sort of behavior.
Is there an smart yet not risky way of keep job B offer on hold for one week? i.e. before I can secure (or discard) job A offer?
Your only options are to delay providing an answer to the first company or to contact the second and say something like "I have an offer from another company, but I would really like working for you better. Is there a way you could provide me with a resolution on our interview, so I can include you in my making of a decision?" Or something like that.
This doesn't necessarily mean it will happen. If their processes are slow, you might still get a response too late.
It might also not be received well (I was more than once in this place and some HR personnel didn't like it. They though I was trying to pressure them. This is a common occurrence in job searching so their way of acting kind of decreased their importance in my decision making process).
So you tell the first company that you will carefully analyze their offer and ask when, at the latest they need a response (they will pressure you to be sooner rather than later :)) and then you contact the second company and kindly ask them to hurry.
Something smarter to do in here?
Not really. You either accept the first offer, try to delay while waiting to receive the second offer, or you refuse the first offer and take you chances with the second. It all depends on how much risk there is in getting refused in the second case, and on how desperate you are for having a job.
Remember the saying "a bird in the hand is worth two in the bush". Try to lay out a logic tree of all possible outcomes and look at all of those outcomes and see with which you are most comfortable with.