It seems in the case of adding a product with an existing ASIN you can actually send a very basic XML request such as this, making sure to include the ASIN:
<AmazonEnvelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="amznenvelope.xsd">
<Header>
<DocumentVersion>1.01</DocumentVersion>
<MerchantIdentifier>MERCHANT_IDENTIFIER</MerchantIdentifier>
</Header>
<MessageType>Product</MessageType>
<PurgeAndReplace>false</PurgeAndReplace>
<Message>
<MessageID>1</MessageID>
<OperationType>Update</OperationType>
<Product>
<SKU>UNIQUE-TO-ME-1234</SKU>
<StandardProductID>
<Type>ASIN</Type>
<Value>B000A0S46M</Value>
</StandardProductID>
<Condition>
<ConditionType>New</ConditionType>
</Condition>
</Product>
</Message>
</AmazonEnvelope>
Essentially though, from what i've read elsewhere it seems that Amazon will attempt to match a product to an existing ASIN according to the data within the _POST_PRODUCT_DATA_ feed even if an ASIN isn't provided. It will use elements such as title, manufacturer, brand, and other product specific information to compare that to their catalog and determine if it is an existing item or a new one to be added. If you do know it already has an ASIN though you can provide a very simple XML feed as shown above.