Installing packages from Wheezy in Squeeze

[James Allsopp <jamesaallsopp@googlemail.com>]

Hello,
Is there an easy way of installing a package (namely virtinst) from 
Wheezy in Squeeze? I've tried variations of
aptitude install -t wheezy virtinst
and
aptitude install virtinst/wheezy

but with no joy, It goes through the start process but says there's 
nothing to update or upgrade
Thanks,
James