You could dump something like that into Flexibility (uses Foundation 3.2) which would give you a number of other potentially useful TV's and pre-installed functionalities but I wouldn't install Flexibility over the top of a working site as it builds example resources and TV's based on a fresh install.
There is also a Foundation package available, that would provide responsive templates and the Foundation files, but doesn't install configured TV's or resources.
The MODX Boilerplate package is also responsive and Bootstrap was also added to package management recently. Look through and see if there's something that you can use. I would suggest installing MODX locally or perhaps in a subfolder on your server to test the packages before you decide. As far as I know, only Flexibility needs to be installed on a fresh MODX install, the others will simply load up a new template and subfolder in assets so they're safe to install wherever.
OR, since your site is already deployed, find your favorite responsive platform, download the files and do it the regular way. You don't really need the packages to make any of the responsive platforms to work. They're just quicker/easier to setup.
I'm partial to Foundation due primarily to the fabulous (simple) grid nesting, but there's plenty other goodies to like as well. Inuit CSS from CSS Wizardry is also nice.
http://foundation.zurb.com/
http://csswizardry.com/inuitcss/inuit.css/demo/
http://twitter.github.com/bootstrap/
https://github.com/anselmh/modx-boilerplate
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