Hmm, the multiadd array just takes product URLs and sends them all to the cart, which I believe means it cannot be used with a product that contains any variants or variations at all.
The multiadd function is an "unsupported hack" that we don't have a lot of experience implementing, we're not programmers here on the support team and even if we were, we couldn't give layman's instructions on how to rewrite everything clearly, so we aren't able to give any detailed support on advanced customizations.
If it is possible at all to use multiadd with variants it would be best to contact a developer who knows how to write that code, experienced developers familiar with our cart can be found here:
http://www.e-junkie.com/ej/developer-directory.htm