Is possible create in the form field to add optional keywords and these appear in the meta keywords?

If you mean the form to submit a new topic, then it would be possible, but is up to some customizations, specially in the database and submission/editing scripts. This is not available by default or any add-on.

In general, if you'd like to make it available to all visitors, then some way you should care about validity of keywords and their format. Only small percent of all forum posters know what it is about and how they should deal with it.

Also, I'm not sure - do you really think such keywords matter a lot? I suppose, Google uses its own algorithms of determining the nature of content, and it doesn't take closely the keywords tag at all.

Meta information regarding your blog is an important part of both search engine optimization and engaging visitors. It's this information that is shown when searching for specific information online. While it's still possible to get a handful of people reading your posts, keywords and meta descriptions are a valuable asset.

Now, meta keywords are not nearly as prominent as they were before. In fact, changes to Google's algorithm have directly affected how meta keywords work. It's to the point that many developers don't utilize the meta information for specific words today.

yes its possible ,some other expert coding reqired.in programmme area.

This is also a very good post which I really enjoyed reading. It is not every day that I have the possibility to see something like this..

As I know Google pay more attention now on the title, while the description is of less importance. So you should pay more attention on composing a title tag, putting the main keyword at the beginning. The content of the post shall be relevant, it can also include keywords.

This wouldn't work for public forums. 99% of those who post on opened forums, do not care about keywords and the structure of their posts (or they may not even know what it is). They just type and click, that's all :-) the forums owner should take care about the rest.