We do indeed carry products that allow the extension of the chimney in order to obtain better draft. Ideally, 15 feet of chimney would be the minimum height, so you are on the right thought path. I do not recommend using the Duraliner, as it is not designed to have more than 2 feet exposed above the chimney and it also is not listed to be mated to flexible liner. Instead, a series of conversion components must be used to make the transition. This consists of a slip connector that the flexible liner would attach to inside the chimney, a metal support plate, a section of Class A rigid chimney, and a cap. Using the length of the slip connector as a gauge, you would cut the liner so that it is slightly recessed into the chimney. The slip connector has a smooth end than can be crimped to meet the I.D. of various liners. The connector would insert into the flexible liner, then band clamp in position. The top of the connector has strapping that would then insert through corresponding slots in the metal support plate (your support plate and cap you have now would be unused). The metal support plate must be secured to the flue with construction adhesive, anchors, or masonry bolts. Ideally, the clay flue tile would be ground down to be flush with the top of the chimney wash for better stability. The section of Class A chimney and cap can then be installed.
Another less costly alternative would be to use the support plate you have, but use a "flue stretcher" type of cap to extend your chimney slightly. The liner would need to be cut down as before, then anchored to the plate. You would then install a product such as the Gelco Stainless Steel 13 in. x 13 in. Base 2 Foot Flue Stretcher
. While this would not add as much height as the other option, it is a more economical alternative.