Add word under cursor to search pattern

Try

:nnoremap <silent> + :let @/ .= '\\|\<'.expand('<cword>').'\>'<cr>n

That will append the word under the cursor to the search register when '+' is hit, and jump to the next occurrence of any searched pattern.

If you wish to extend it to the visual mode, (as it could be done to n_star), you have

:vnoremap <silent> + <c-\><c-n>:let @/ .= '\\|'.escape(lh#visual#selection(), '/\^$*.[~')<cr>n

With lh#visual#selection() to fetch the current selection, and escape() to neutralize some active characters in regexes. v_CTRL-\_CTRL-N being a safe and silent escape sequence.

Try something like:

1. * (to search for a word)
2. move somewhere else
3. :let @/=@/.'\|\<'.expand("<cword>").'\>' this appends to the previous search pattern the current word under the cursor) with some delimiters (\| and the word boundaries...)
4. if you want to, set up a hotkey for it, like: nnoremap <F4> :let @/.='\\|\<'.expand("<cword>").'\>'<CR>