If a topic (only one page) has been accepted previously in poster session of a conference / workshop, is it possible to submit the long version of the same topic (15 pages) to another conference? (particularly in field of computer science)
Based on your question, I can see no issues with that.
1. Yes. Note that 'Buffy's answer regarding self-plagiarism is a fair point though I think the additional 14 pages probably helps mitigate this issue substantially (as pointed out by 'Jon Custer'). Citing the previous (shorter) work is a good idea and in general can't really hurt you.
2. You can always double-check if you're worried. You can always submit with an attached letter (or include in email) that a small portion of the material was published or presented in/at [X]. Then no one would think you're trying to be sneaky.
Caveat: As always, these things are situation dependent (field, conference, etc). I would further defer to a Computer Science answer if it disagreed with mine.
Only the conference committee can really answer that, but I offer two suggestions. These apply even if the conference committee sees no problem.
There is a concept called self-plagiarism, that I think is a bit stupid, but it exists and many consider it a serious issue. You should, at a minimum cite your own earlier work (the poster) just as you would cite anyone else and quote it appropriately.
The second suggestion is that, while you still cite the poster work, you re-write it, incorporating the important ideas rather than quoting it directly.
But you will run in to trouble with some if you just use it without comment even though it is your own work.
I doubt that the field matters much. My field is also CS, however.