Question: There are some keywords I search frequently for throughout my project. I would like to use search like this: I put some keyword into the "register", after a while (I may or may not use regular search during that time) I want to search for that keyword in the currently open file. So I load it from the register and then `n`/`N` movements work like usual, just that they search for the keyword I have loaded.

Is that possible?

Answer: 
You can do *exactly* what you want, actually.

Vim has 26 "named registers" (a-z) which you can use for recording macros or storing text. You can use these registers as part of commands, for example to copy the word under the cursor into register 'a', type `"yiw` in normal mode. You can also assign to them directly, for example to put the word "snarfblat" into register 'b' use `:let @b='snarfblat'` (and press `Enter`).

Once you have text in a register, you can retrieve it in a search using `Ctrl`+`R` and then pressing the register you want to retrieve. So in your case to search for the contents of register 'a' simply start a search with `/`, press `Ctrl`+`R` followed by `a` to insert the register text, then press `Enter` to do the search as normal.

See `:help registers` for details.
You could put the keywords in a file and then `:source filename`. But generally when searching it's easier to hit `/` and then use the up/down arrow keys to use previous search terms.