mxmlc considers a swc a library (it's actually a zip file that you can examine) so you'll want to do something like the following:
mxmlc -compiler.library-path /path/to/your/swc
You can get help with mxmlc -help <TERM>. For example:
$ mxmlc -help library-path
Adobe Flex Compiler (mxmlc)
Version 3.3.0 build 4852
Copyright (c) 2004-2007 Adobe Systems, Inc. All rights reserved.
-compiler.external-library-path [path-element] [...]
alias -external-library-path
list of SWC files or directories to compile against but to omit from
linking (repeatable)
-compiler.library-path [path-element] [...]
alias -l
list of SWC files or directories that contain SWC files (repeatable)
-runtime-shared-library-path [path-element] [rsl-url] [policy-file-url] [rsl-url] [policy-file-url]
alias -rslp
(repeatable)