You could alter the bibliography style. For verbose-trad2 it is defined primarily by standard.bbx. The code below will incorporate volumes, volume, maintitle and part fields for @report in a manner consistent with @book. Note that @techreport is just an alias for @report with type = {techreport}.
\documentclass{article}
\usepackage[style=verbose-trad2]{biblatex}
\usepackage{filecontents}
\DeclareBibliographyDriver{report}{% based on report and book drivers from standard.bbx
\usebibmacro{bibindex}%
\usebibmacro{begentry}%
\usebibmacro{author}%
\setunit{\labelnamepunct}\newblock
\usebibmacro{maintitle+title}
\newunit
\printlist{language}%
\newunit\newblock
\usebibmacro{byauthor}%
\newunit\newblock
\iffieldundef{maintitle}
{\printfield{volume}%
\printfield{part}}
{}%
\newunit
\printfield{volumes}%
\newunit\newblock
\printfield{type}%
\setunit*{\addspace}%
\printfield{number}%
\newunit\newblock
\printfield{version}%
\newunit
\printfield{note}%
\newunit\newblock
\usebibmacro{institution+location+date}%
\newunit\newblock
\usebibmacro{chapter+pages}%
\newunit
\printfield{pagetotal}%
\newunit\newblock
\iftoggle{bbx:isbn}
{\printfield{isrn}}
{}%
\newunit\newblock
\usebibmacro{doi+eprint+url}%
\newunit\newblock
\usebibmacro{addendum+pubstate}%
\setunit{\bibpagerefpunct}\newblock
\usebibmacro{pageref}%
\usebibmacro{finentry}}
\begin{filecontents}{\jobname.bib}
@Techreport{padhye,
author = {Padhye, Jitendra and Firoiu, Victor and Towsley, Don},
maintitle = {A Stochastic Model of TCP Reno Congestion Avoidance and Control},
institution = {University of Massachusetts},
number = {99-02},
location = {Amherst, Mass.},
date = {1999}}
@Techreport{padhye:mv,
author = {Padhye, Jitendra and Firoiu, Victor and Towsley, Don},
maintitle = {A Stochastic Model of TCP Reno Congestion Avoidance and Control},
institution = {University of Massachusetts},
volumes = {2},
number = {99-02},
location = {Amherst, Mass.},
date = {1999}}
@Book{knuth:ct,
author = {Knuth, Donald E.},
title = {Computers \& Typesetting},
volumes = {5},
publisher = {Addison-Wesley},
location = {Reading, Mass.},
date = {1984/1986}}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\noindent
Technical report: \cite{padhye} \\\\
Multi-volume technical report: \cite{padhye:mv} \\\\
Multi-volume book: \cite{knuth:ct}
\printbibliography
\end{document}

@techreportwill take the fieldvolumesin the current version of biblatex or in a future release? If you wonder about the current release you can just check the manual or try it. – N.N. Jul 8 '11 at 17:27