Aren't Arch packages pretty much the same as Slackware packages, with the possible exception of the .PKGINFO file on Arch vs the SlackDesc file on Slackware? Maybe someone can verify this.
If they are the same (or nearly the same) and there's not an Arch-specific tool to do the conversion, then have a look at the deb2tgz script here (there may be others): http://code.google.com/p/deb2tgz/downloads/list
Note that the slackware .tgz extension just indicates that it's a slackware package - the archive itself is still a .tar.gz inside.
A Slackware package is just a binary tar archive ready for installation, as is (AFAIK) a Arch package. You may need to fiddle with the script or tune it a bit to suit your needs, like maybe to alter the finished package extension, or add a .PGKINFO file, but it's close already.
P.S. - note that both Slackware and apparently Arch are using/moving to the XZ compression, so a particular version of the script may or may not create packages with one or the other extension.
Best of luck,