1. a footer like
   
   is possible with Layout->Document->Pagestyle->fancy and

   \usepackage{calc}
   \renewcommand{\headrulewidth}{0pt}
   \newlength{\fminilength}%
   \setlength{\fminilength}{\textwidth-2\fboxsep-2\fboxrule}%
   \pagestyle{fancy}
   \lfoot{\noindent\fbox{%
       \begin{minipage}{\fminilength}%
         \ttfamily\upshape Text A\hfill Text B
       \end{minipage}%
     }}
   \cfoot{}

2. to get a header for the letter class for following pages you can activate Layout->Document->Pagestyle->headings. Than you'll get the default. An own style like
   
   is possible with

   \newlength{\mySendto}
   \renewcommand\@oddhead{%
     \settowidth{\mySendto}{\toaddress}%
     \parbox[t]{0.45\columnwidth}{%
        \begin{tabbing}
           \textbf{\headtoname{}:} \=\toname{}\\
            \>\parbox[b]{\mySendto}{\toaddress}
        \end{tabbing}%
       }\hfill\parbox[t]{0.45\columnwidth}{\flushright\pagename{} \thepage}%
   }

   Together with the babel package you also get the language specific sendto page.
3. An example for a header with different parboxes.

   

   

Double headrule

Colored headrule