If the new column is going to have the same value on each line (i.e. your initial yank was on a single line), then is probably just easier to visually block select the space between the username and password column and type A (or I) to start appending (inserting). When you return to normal mode (Esc), Vim will append the text to the rest of the original selection, too.
If you really need to do a repeating pattern, then you could use a function and mapping like this:
:function! PutRegFilledToRange() range
: let l:fill = ''
: let l:lines = getreg()
: let l:lineStart = 0
: let l:linesLen = strlen(l:lines)
: let l:lineIndex = 0
: let l:linesNeeded = a:lastline - a:firstline + 1
: while l:lineIndex < l:linesNeeded
: let l:lineEnd = matchend(l:lines, "\n\\|$", l:lineStart)
: let l:line = strpart(l:lines, l:lineStart, l:lineEnd - l:lineStart)
: if l:lines[l:lineEnd - 1] != "\n"
: let l:line = l:line . "\n"
: endif
: let l:fill = l:fill . l:line
: let l:lineStart = l:lineEnd < l:linesLen ? l:lineEnd : 0
: let l:lineIndex = l:lineIndex + 1
: endwhile
:
: normal gv
: " This probably does not make much sense in any mode
: " other than block modes, but we will try to cope.
: if mode() == "\<C-V>" || mode() == "\<C-S>"
: let l:newMode = "\<C-V>"
: elseif mode() == 'v' || mode() == 's'
: let l:newMode = 'v'
: else
: let l:newMode = 'V'
: endif
: let l:origReg = getreg('')
: let l:origRegType = getregtype('')
: try
: call setreg('', l:fill, l:newMode)
: execute 'normal ' . v:count1 . 'p'
: finally
: call setreg('', l:origReg, l:origRegType)
: endtry
:endfunction
:vmap _f :call PutRegFilledToRange()<CR>
Put in your .vimrc, or some other file (and run :source /path/to/file). Or paste it directly into a Vim session.
To use it:
C-V{motion}y: visually block select and yank the text you want to use to repeatedly put
{motion}C-V{motion}: select the block you want to replace
_f: fill the selected block with repeated lines of the current register ("a_f fills block with lines from register a)