How to add a paper to google scholar as "to appear"?
Meanwhile, I would like to cite mentioned paper in another paper. How do I do it? I want it to be added as &quot;to appear in ...&quot;

Cite the paper as you would any IEEE conference paper, omitting any information that you don't know (e.g., page number) and tagging &quot;to appear&quot; on the end.

Can I add the article manually to google scholar?

If you want.

If I do so, what happens if the proceedings out eventually? Will it be merged automatically?

It may or may not be automatically merged, we cannot predict how Google's algorithms operate.

Because of copy right I can not publish the pdf in preprint,

Actually, you likely can, since many publishers let you publish preprints on personal websites.