What are other options?

My first guess would be that either they honestly overlooked it and need the things you don't have; or they noticed the omission, but are still interested in you - maybe for a slightly different position, or for learning on the job.

How should I approach that during the interview ("sell it")?

Be as natural as you can be. View it as an occasion for networking, or to make a small foray into an unknown area. Don't view it as an interview as such; or rather an interview of your general character or approach to learning.

Do not try to get some superficial know-how in the week leading up to the interview. They will most certainly catch up to that, and if you try to pass it off as more than "I've googled it", you'll pretty much be disqualified for seeming slightly dishonest.

Should I be assertive about not having experience in one of the fields they expect experience in and stress expertise in other fields not mentioned in the ad?

"Assertive" and "stress" sounds quite laborious. Of course you should let them know, and maybe make sure that what's already in your CV is repeated during the talk. Some managers may not study the CV in-depth before the meeting and may have forgotten about this missing item. But I would not make a huge issue of it.

Can I do anything to increase my chances of getting the job? I want it, but don't fulfill the conditions.

Be yourself and hope that they see something that they like. Obviously, try to find out more about the job during the meeting, and if you see anything in the area you are already experienced in, make sure they know what you have to offer.

If, near the end of the interview (or the part where you talked about your experiences), you are not quite convinced that the issue has come up sufficiently, you may ask a last question, something like "by the way, you list A, B and C as prerequisites - I bring C, but you should be aware that A and B are not really my expertise". And see how they react.

What are other options?

My first guess would be that either they honestly overlooked it and need the things you don't have; or they noticed the omission, but are still interested in you - maybe for a slightly different position, or for learning on the job.

How should I approach that during the interview ("sell it")?

Be as natural as you can be. View it as an occasion for networking, or to make a small foray into an unknown area. Don't view it as an interview as such; or rather an interview of your general character or approach to learning.

I wouldn't like to waste a week or more preparing

A short period of study to get a basic overview is of course important, but do not try to get some superficial know-how by studying for a week or more leading up to the interview. They will most certainly catch up to that, and if you try to pass it off as more than "I've googled it", you'll pretty much be disqualified for seeming slightly dishonest.

Should I be assertive about not having experience in one of the fields they expect experience in and stress expertise in other fields not mentioned in the ad?

"Assertive" and "stress" sounds quite laborious. Of course you should let them know, and maybe make sure that what's already in your CV is repeated during the talk. Some managers may not study the CV in-depth before the meeting and may have forgotten about this missing item. But I would not make a huge issue of it.

Can I do anything to increase my chances of getting the job? I want it, but don't fulfill the conditions.

Be yourself and hope that they see something that they like. Obviously, try to find out more about the job during the meeting, and if you see anything in the area you are already experienced in, make sure they know what you have to offer.

If, near the end of the interview (or the part where you talked about your experiences), you are not quite convinced that the issue has come up sufficiently, you may ask a last question, something like "by the way, you list A, B and C as prerequisites - I bring C, but you should be aware that A and B are not really my expertise". And see how they react.