Weddingbee boards

I bought this DB gown: http://www.davidsbridal.com/Product_Satin-Strapless-Trumpet-Gown-With-Ribbon-Waist-WG9871_Bridal-Gowns-Shop-By-Silhouette-Mermaid-Trumpet but am in love with the look of lace cap sleeves with a keyhole back, so was thinking of adding that onto this dress.  The DB alterations lady suggested against it saying that not only would I have to find my own lace to bring to them, but that it would ruin the look of the dress.  Does anybody have any experience with adding such things onto a strapless dress and any pictures of the final product?  The other option is to add the DB cap sleeves, such as: http://www.davidsbridal.com/Product_Cap-sleeve-with-beaded-floral-motif.-CAP2_Accessories-Sashes-and-Coverage-Options-All-Sashes-Coverage-Options but this is not lace and I wouldn't have that romantic look of a keyhole back.  Any ideas, hive?  Thank you!