Haven't brewed a Dubbel yet, but I was under the impression you wanted fermentation to be over with before moving to secondary, so not sure of the purpose of adding additional yeast, unless Dubbel's have a knack for killing off yeast somehow. Secondary fermentation I think is a bit of a misnomer in itself, it's just a secondary vessel to aid in clearing and flavor additions, as far as I knew. I can't imagine it will do anything bad to it, but it sounds like you'd be wasting a smack pack.
Kegs: Tri-Peppered Saison and Sunset Orange Kolsch
Future:Something new, SMaSH perhaps?
2013 Previous: Sunrise Honey Rye Pale Ale, Dark Skies Black IPA, Sunset Orange Kolsch, All Hallow's Eve Pumpkin Ale, Raining Hops and Dogs IPA
Dark Skies Black IPA - 2nd Place at a local comp. Score of 39 from a BJCP judge
GT Keezer Build