The whole tikzposter is essentially one tikzpicture. Further, the class defines a coordinate bottomleft at the lower left corner of the paper, so you could probably just add something like
\node [above right,outer sep=0pt,minimum width=\paperwidth,align=center,draw,fill=blue!30] at (bottomleft) {Some text at the bottom};
just before \end{document}.
Complete example:
\documentclass[a0paper]{tikzposter}
\author{Someone}
\title{Something}
\begin{document}
\maketitle[width=0.95\textwidth,linewidth=0pt]
\node [above right,
outer sep=0pt,
minimum width=\paperwidth-2*\pgflinewidth,
minimum height=7cm,
align=center,font=\Huge,
draw,fill=blue!30,
ultra thick] at ([shift={(0.5*\pgflinewidth,0.5*\pgflinewidth)}]bottomleft) {Some text at the bottom};
\end{document}

drawandultra thickfrom the node options. You can also remove-2*\pgflinewidthfrom theminimum width, and[shift=...]from the coordinate. – Torbjørn T. May 12 '16 at 14:31