You can either remove the restriction line from the plugin source file and re-compile it in C++. The restriction line matches the stock model file name with the file size of the stock model, so theoretically, if you had a non-stock sod, you could rename it to the name of a stock sod with the exact same file size.
You can also use Conversion Versions program located on this page:
http://www.frost-works.com/stc/conversionversions/CVArchive/CV0.7.5/index.htmlto directly convert the sod to mod. After it is in mod format, it becomes importable.
There is also Lithium Unwrap which can import Armada 1 sod files:
http://files.seriouszone.com/download.php?fileid=198Be careful asking this question in Armada forums, it is apparently a touchy subject with them.